Hi Bli2016,
I am so glad you asked this question because it leads to discussing a really important distinction.
An
argument is one or more premises given in support of a conclusion + that conclusion. Powerscore uses the metaphor of a house--the walls are premises and the roof is the conclusion. The argument is the whole house, composed of premises (including possibly subsidiary conclusions) and a main conclusion.
In your question, you say:
I recognized that the stimulus is trying to disprove the argument that evolution always optimizes the survival of an organism with the example of the moose.
But, to be accurate, you need to replace the word "argument" with claim, assertion, statement, etc.
(A) would work as a correct answer if the stimulus went through a complete argument that concluding that evolution always optimizes, then cited the example of moose antlers.
Great question!