Hi Jennie,
So this is a stimulus that is missing a supporter assumption, requiring us to tie something new in the conclusion to the premises. Here, the new element in the conclusion is "engagement," which isn't present in any of the premises. (E) ties engagement to amount of time spent looking at a work, clearly tying the conclusion to the premises. (D), however, only deals with whether someone who takes a snapshot (which are only some of museum goers) is likely to look at that picture, which is ultimately irrelevant--we still need to know that time spent on a piece shows the level of engagement with that piece. To test this, we can use the Assumption Negation technique:
Snapshots often looked at
People not necessarily less willing to engage with art
This would perhaps weaken the argument, but it doesn't work to 100% negate the conclusion, so it doesn't work as a necessary assumption.
Contrast this with (E):
Time spent not a reliable measure of engagement with art
People not necessarily less willing to engage with art
As we see, this eliminates the only premise for the conclusion and effectively negates the argument.
Hope this clears things up!