Before sometimes, the monkey jumped . . . . . . the river.

Answer: Option B

The sentence describes the monkey's movement. The preposition "into" indicates movement from the outside to the inside of something. In this context, it shows the monkey moving from a position outside the river to a position inside the river (presumably by jumping in).

Why other options are incorrect:
A (on): "On" indicates a position on the surface of something. The monkey didn't jump onto the surface of the river; it jumped into the water.
C (near): "Near" indicates proximity. This doesn't describe the monkey's action (jumping) or its location after the jump. The monkey jumped *into* the river, not just near it.
D (upon): "Upon" is a more formal synonym of "on," and suffers from the same problem as "on." It implies a position on the surface, not an entry into the river.

Therefore, only "into" accurately reflects the action and location described in the sentence.