Question

complete the sentence with the correct correlative conjunction.
lets look up __ running __ biking is the most popular form of exercise in this country.
both...and
not only...but also
whether...or

Explanation:

The sentence is asking to find out which single exercise (running or biking) is the most popular, so we need a correlative conjunction that expresses a choice between two options. "both...and" would refer to both as popular together, "not only...but also" adds emphasis to including both, while "whether...or" is used to introduce two alternative possibilities for a comparison or choice, which fits the context of determining which one is the most popular.

Answer:

whether ... or