Answer:
D. K star
Explanation:
Stars are classified into different groups according to their peak wavelength and their surface temperature.
In particular, we have the following group of stars, which correspond to the following surface temperatures:
Group O - Temperature > 25,000 K
Group B - Temperature 11,000 - 25,000 K
Group A - Temperature 7,500 - 11,000 K
Group F - Temperature 6,000 - 7,500 K
Group G - Temperature 5,000 - 6,000 K
Group K - Temperature 3,500 - 5,000 K
Group M - Temperature < 3,500 K
So among the options given, the star with the coolest surface temperature is star in group K.
