Which of these described climate rather than weather?
a. wind speed changes as a storm moves through an area
b. the temperature decreases in a slow-moving cold front
c. annual high temperatures in the summer have increased over many decades
d. the rainfall this year was greater than the rainfall this year

The answer is:  [C]: 

             "Annual high temperatures in the summer have increased over
 many decades."

Note:  Refer to:  "...have increased OVER MANY DECADES..." ;  whwic indicates "climate" rather than "weather".  Note that "weather"' may change, however slightly, from "day to day".  The climate refers to longer-term, overall trends.
.

Your answer is going to be b.