The correct answer is “to”.
The word “to” is used to express preference. For example, “I prefer tea to coffee” means that I like tea more than coffee.
The word “than” is used to compare two things. For example, “I like tea more than coffee” means that I like tea more than coffee.
The word “over” is used to express superiority. For example, “I prefer tea over coffee” means that I think tea is better than coffee.
The word “against” is used to express opposition. For example, “I am against drinking coffee” means that I do not like coffee.
In the sentence “I prefer tea to coffee”, the word “to” is used to express preference. The speaker is saying that they like tea more than coffee.