You might begin this response by explaining that this question is considering an a priori proof for the existence of God. You could explain that, for Anselm, merely understanding the statement ‘God exists’ properly would mean that you should accept it as a statement of truth. You could then outline one or more versions of the argument moving, for example, from the idea of a being greater than which cannot be conceived through the reductio ad absurdum to the conclusion that God must exist in reality as well as in the mind.
Alternatively you could explain the use of logically necessary existential propositions and their relationship to factually necessary existential propositions, exploring why Anselm sought to find a proof for God’s existence that was prior to and not dependent on existence.
You might also put your explanation in the context of Anselm’s own thinking in terms of seeking to see if his faith is rational and would fit into a logical argument. He did not himself need a proof for the existence of God.
Here you are being asked to assess the extent to which Anselm was successful in his assertion that belief in God was a logical necessity. You could evaluate the extent to which the reductio ad absurdum is a valid move in this argument, assessing whether or not it is more than some sort of philosophical sleight of hand.
Alternatively, you might explore the extent to which the argument seems to be valid, in logical terms, but is not true as one or other of its premises is false.