Answer:
All of the above
Step-by-step explanation:
Although the option isn't listed in the question, I see that you have commented that the fourth option, D. All of the above, exists.
The word effect can be both a noun and a verb. As a noun, it is often mixed up with the verb to affect (effect vs affect), but it can also be a verb meaning to cause something to happen. We could write the first sentence differently as: The student council wants to cause (= to effect) a change in the dress code.
Effect as a noun is used in the third sentence, and it means a change that is a result or consequence of an action (Playing sports can cause a change in (= have an effect on) students' academic performance).
In the second sentence, we have the verb to affect, which means to have an effect (Receiving a good grade will have an effect on (= affect) the student's motivation).