The phrase is incomplete and needs more context. You need to add the word after "à" to understand what it means. For example:
* Quel est le verbe écrit à l'infinitif? (What is the verb written in the infinitive?)
* Quel est le verbe écrit à la troisième personne du singulier? (What is the verb written in the third person singular?)
* Quel est le verbe écrit à l'imparfait? (What is the verb written in the imperfect tense?)
Let me know the full phrase, and I can give you a more specific translation.