Which of the following statements is false?
a. air temperatures over the ocean remain fairly constant throughout a 24-hour cycle.
b. air temperatures over the land can vary significantly throughout the day
c. air temperatures over land reach their highest values in the mid-afternoon.
d. air temperatures over land are always higher than air temperatures over the ocean.

The range of temperatures in a 24 hours cycle is different in the air over the land than in the iar over the ocean, due to the different specific heats of air and water.

Given the high specific heat of water the temperatures over the ocean remain fairly constant throughout a 24 hours cycle day, the the option A. is true.

During the day the land absorbs a great quantity of heat that is released during the night, which leads to a hight decrease of temperature in the temperature of the air over the land during the night. Then the option B and C are also true. 

This higher change in temperature in the air over the land than in the air over the ocean may leads fo the falsity of the option D. because, the air temperatures over the land may reach lower temperatures than the air over the ocean dufing the nigh.

Then, the answer is that the statement held by the option D is the FALSE one.

D.air temperatures over land are always higher than air temperatures over the ocean.