To answer your question, only をしている is correct and がしている is wrong in this example.
Actually, those two are confusingly similar but unrelated. What the verb する means in the two idioms are different things.
- がする → to strike the (i.e. your) sensation
- をする → to wear some (persistent) traits
がする is to describe an ephemeral sense (stimulus) that can appear and disappear. For example, there is no sound when you hear nothing, and no odor when you smell nothing, because those sensations are direct interaction between your organ and the medium. Usually you can understand Xがする as "I sense X".
誰もいない部屋から物音がした I heard something from an empty room
とてつもなく悪い予感がする I have (= get) an extraordinarily bad feeling
However, we don't perceive colors likewise. Although today we know that everything is quantum, most people still take it for granted that a red apple never ceases to be red if you look aside. In other words, we believe that the redness belongs to the apple, not to the light (unless it's a laser ray) in the usual sense. That makes がする unusable in that sentence. But if you had some eye disease and saw a red color for no reason, 赤い色がする would make sense.
Some previous answers tell that をする is for visual stimuli, but this is somewhat superficial. We do say:
優しい性格をした人 a person who has a gentle personality
彼女は高い声をしている she has a high voice
どんな育ち方をしたらこんな絵が描けるのか what kind of life makes (= experiencing what life history) him/her create such a drawing
Of course, 彼女は高い声をしている doesn't mean she emits a high frequency wave 24 hours, but that she does when she speaks. 高い声がしている means that you hear a voice persisting as long as it may.
One grammar point is that, since をする depicts a sort of state rather than action, the dictionary form is virtually unusable. You should always say をしている and をしていた as the main verb of the sentence.
By the way I don't think the original quiz is very well thought out. You can easily find a context where A is valid, and if you stretch your imagination, D actually might be.
→ Mori-san makes their fish have a beautiful color (painted?)
→ maybe some alien fish that shines (emits light) like a mirror ball?