Sea Surface Temperatures (SST) are warmest:

A. at the poles where long days during summer warm water
B. along the equator where the Sun's rays are most direct
C. at the subtropics because descending air promotes clear skies
D. where oceans are deepest allowing for sufficient mixing

Option C, At the subtropics because descending air promotes clear skies.

Explanation:

The temperature of water at  the surface of ocean is known as Sea surface temperature and is affected by the productivity of ocean, pollution level of sea water , global climate change and geographical location.  

In the sub-tropical region especially in the western part of the tropical Pacific. , the rainfall is heavy that results into  atmospheric convergence and deep atmospheric convection leading to warmer sea water that is confined to the western part of the tropical Pacific.  

Hence, option C is correct