Many is a pronoun which is always plural. This is because if you have many of something, you must have multiple. You cannot say "many" and then have a singular noun.
Each is not the correct answer because it can be singular. For example, "each person in the room was standing."
Neither is not the correct answer either. For example, "neither cookie was made by her mother."
Every is not the correct answer. For example, "every day I try to be happy."
