Whom should be used to refer to the object of a verb or preposition. Who should be used to refer to the subject of a sentence. Here's a nice trick. If you can replace the word with "he" or "she", use "who". If you can replace it with "him" or "her", use "whom."
Let's look at the sentences:
1. Who loves science?
- We could say, "He loves science." Therefore, who is correct.
2. The class whom has the best projects will go on a trip.
- We can use whom or who with plural nouns to. However, whom is not correct in this sentence. Since the word is referring to the subject of the sentence (the class), it should be who.
3. Who is this package for?
- We could say, "This package is for him." Therefore, we should use whom instead of who.
4. Jayden is the boy whom scored the goal in the big game.
- We could say, "He scored the goal." Therefore, we should use who instead of whom.