Answer: A. The ocean has a higher heat capacity
Explanation: Heat capacity refers to the amount of heat energy necessary to produce a change in temperature within a substance. Different materials and molecules have differing heat capacities due to differences in chemical structure. An object with a high heat capacity is more difficult to heat up and cool down, while objects with low heat capacities easily have their temperatures changed.
The ocean is mostly made up of water (or H2O). As it turns out, water has a very high heat capacity compared to most other molecules, thus making the ocean more resistant to temperature changes. As a point of comparison, water has a heat capacity of 4.186 joules per gram/Celsius. Meanwhile, air's heat capacity is only 1.02 joules per gram/Celsius. Since air's heat capacity is so much lower, the atmosphere's temperature changes much more readily than the temperature of the ocean.
Thus, the ocean heats up and cools down more slowly than the atmosphere because the ocean has a higher heat capacity than the atmosphere.