always remember them in order with formula, SITS=AEEI
and the two on the END are AD-DUCTION

So this question is describing a guy having difficulty externally rotating his arm while the forearm is flexed. He is able to supinate while his arm is extended (because of the supinator muscle and biceps). In the rotator cuff, only the infraspinatus performs external rotation, so that is the best choice.

Biceps works fine here

Subscapularis performs internal rotation.

Supraspinatus performs abduction

Triceps provides extension of forearm.