He gave a talk . . . . . . . . the causes of dengue fever.

Solution (By Examveda Team)

In this sentence, "on" is the correct preposition to use.

"He gave a talk on the causes of dengue fever" means he delivered a speech or presentation about the causes of dengue fever.

Using "of" would imply belonging or connection, as in "the causes of dengue fever," which describes what causes dengue fever. However, it does not fit in the context of giving a talk.

"For" would suggest purpose or benefit, but it does not fit here because the talk is not intended to benefit the causes of dengue fever.

"Off" is incorrect as it generally implies separation or distance and does not make sense in the context of delivering a talk.

Therefore, "on" is the appropriate preposition to use when referring to the subject or topic of a talk.