A "U-shaped" distribution for a trait, with high frequencies of individuals who exhibit extreme values for a trait (and few individuals with medium values), is most likely to be caused by selection

Disruptive Selection.

Explanation:

Disruptive selection, also known as selection diversification, explains shifts in population genetics that prefer extreme values for a trait over intermediate values. In this case, trait variation increases, and the population is split into two different classes. So, in the given case where A "U-shaped" distribution for a trait, with high frequencies of individuals who exhibit extreme values for a trait (and few individuals with medium values), is most likely to be caused by Disruptive selection.

the answer would be..

b. sucking