Fill in the blank with a suitable modal:
We _________ to stop pollution.
A verb is used to express an occurrence, action, or state. It is the grammatical centre of the predicate in a sentence.
An auxiliary verb is a verb which gives is used in the formation of tenses, moods, tones, etc.
Option D is correct as the sentence and infinitive indicates the use of a modal that indicates necessity.
The other options are wrong as we can not use them before to.
The correct answer is D) ought