More than half of turkey’s population lives along the country’s coastal plains because
a. its major cities are located there.
b. its electric power plants are there.
c. the sources of fresh water are there.
d. the climate is milder and wetter ther

The correct answer is the last one: d. the climate is milder and wetter there.

Central Turkey is very cold and dry, especially in the winter, and this means that life is more pleasant on the coast. This also means that the major cities are located there: but that's not the primary reason, rather a consequence of the primary reason.