B

To understand the formation of a primary rainbow, we need to analyze the path of light through a raindrop.
Step 1: When sunlight enters the raindrop, it undergoes refraction as it passes from air to water. This is the first refraction.
Step 2: After refraction, the light travels inside the raindrop, where it undergoes one internal reflection.
Step 3: Finally, before exiting the raindrop, the light undergoes a second refraction as it moves from water to air.
Therefore, the formation of the primary rainbow involves one internal reflection and two refractions.
Thus, the correct answer is option B.