Literal Translation:
* Why do the students contest? This is the most literal translation, but it doesn't really make sense in English unless we know what the students are contesting.
More natural English translations:
* Why are the students answering? This assumes that "contestan" refers to the students responding to a question or prompt.
* Why are the students arguing? This assumes that "contestan" refers to the students disagreeing or debating something.
* Why are the students protesting? This assumes that "contestan" refers to the students expressing opposition to something.
Context is key! To understand the true meaning of "por que contestan los estudiantes," we need more information. What is the context of this sentence? Who is speaking it? What is the situation?
For example, if you are asking about a specific question in a test, then "why are the students answering?" is likely the best translation. But if the students are participating in a debate, then "why are the students arguing?" would be more appropriate.