The sound of the running water of the stream had a pleasantly . . . . . . . . effect on me.

sonorous
amusing
loud
somnolent

The correct answer is A. sonorous.

Sonorous means having a deep, rich sound. The sound of running water is often described as sonorous because it is a continuous, low-pitched sound that can be quite soothing.

The other options are incorrect because they do not accurately describe the sound of running water. Amusing means causing laughter or amusement. Loud means having a high volume or intensity. Somnolent means causing or tending to cause sleepiness.