Sinking air warms by this process:

The correct answer is A. Compression.

When air sinks, it is compressed by the air above it. This compression causes the air molecules to move closer together, which increases their temperature.

Option B is incorrect because expansion causes air to cool. When air expands, the air molecules move further apart, which decreases their temperature.

Option C is incorrect because condensation is the process of water vapor in the air turning into liquid water. This process does not cause air to warm.

Option D is incorrect because friction is the force that opposes the relative motion of two objects that are in contact with each other. This force does not cause air to warm.