Step 1: Understanding the Concept:
This question tests the correct use of prepositions, specifically prepositions of movement.
Step 2: Detailed Explanation:
The verb "jumping" indicates motion. The sentence describes the action of moving from a location outside the pond to a location inside the pond.
Let's analyze the prepositions:
in: Indicates a state of being inside something (e.g., "The boys were swimming {in} the pond."). It does not show movement from outside to inside.
between: Used to describe the space separating two things (e.g., "The path is {between} the trees.").
upon: A more formal version of 'on', meaning on top of something (e.g., "He sat {upon} the throne.").
into: Used to show movement toward the inside of a place or container (e.g., "He walked {into} the room.").
Since the boys are moving from outside to inside the pond, the correct preposition is 'into'.
Step 3: Final Answer:
The correct preposition to complete the sentence is 'into'.