Sure, that's a good question. As a reference, did you check out the online explanation for this question? That's always a good place to start when you run into problem on any homework question. That said, let's look at this more closely.
Like many Must Be True questions, this isn't an argument; it's just a set of facts. And very early on, you start seeing a lot of conditional indicators: "presupposed the existence," "unless," etc. the key then is to see the connection between the relationships. For the purposes of clarity, I'll diagram what appears in the stimulus:
- The first sentence can be broken down pretty easily:
"For the writers who first gave feudalism its name, existence of feudalism presupposed the existence of a noble class"
Feudalism 
Noble Class
So, for the writers who first named it, a noble class was a necessary condition.
The second sentence contains a compound necessary condition:
"there cannot be a noble class, properly speaking, unless both the titles that indicate superior, noble status and the inheritance of such titles are sanctioned by law"
Titles
Noble Class +
Inheritance Law
Ok, so to have a noble class we need two other things. Note that these first two diagrams can be combined to create a longer chain, indicating that feudalism as defined needs these two things (titles and inheritance law).
The third sentence indicates that if the above is true, we have a problem of sorts:
"Although feudalism existed in Europe as early as the eighth century, it was not until the twelfth century, when many feudal institutions were in decline, that the hereditary transfer of legally recognized titles of nobility first appeared."
This is the moment you have to add things up and realize there is an issue: feudalism is stated to have existed in the 8th Century, but inheritable transfer of titles by law did not appear until the 12th Century? That doesn't work with the chain that we have because feudalism requires inheritable titles recognized by law, but that didn't come until four centuries later. So, something is messed up here.
As you know, the answer that describes the situation that something is wrong is answer choice (A). (A) is a pretty unusual answer choice, but then again, this is a pretty unusual stimulus. Essentially, that third sentence states that in the 8th Century the sufficient condition was in effect, but also that the necessary condition did not come into effect until the 12th Century. That's an untenable situation, and if the necessary condition does not occur, but feudalism really did exist, then "the writers who first gave feudalism its name" must have had it wrong, and their definition isn't an accurate picture of history.
So, a tough problem overall, and rather unique. But, this is further exposure to the many ways they can manipulate these statements, and it is a topic that won't go away any time soon. We'll be looking at all sorts of variations on these themes, and one of the things you'll see in the homework is that we often toss in more advanced ideas before you've really encountered them in full. This isn't done to frustrate you, but rather to make you think about the options and because studies have shown that prior exposure to an idea before it is more fully taught makes it easier to absorb over the long run.
Please let me know if that helps. Thanks!
When you move on to the problems, a lot more "noise" creeps into the equation, and then you have to start distinguishing between what is relevant and what is not. that's a learning process, and you will get better over time via practice and further exposure. That said, can you give us an idea of some of the problems you have missed? Even just one or two examples--along with what you were thinking--will allow us to see if there is an underlying root cause that we can identify and eliminate.