What is the definition of "Federal System"? And how is it important?

"if one thing increases, some other thing must decrease" it says in the wiki description for a trade-off. So therefore i would go for an answer where he will be sacrifying some interest time for homework. All answers that aren't compromising I will rule out right away, which is (C.) and (E.). Since reducing tv time by one hour for homework won't completely make him give up on the total of 2 hours TV time it isn't (B.). Now since he has to meet the criteria given by his teacher to solve the whole of math homework I'd rule out (A.). Which only leaves (D.) as the right answer, since he sacrifices time on gaming for homework.