Don't let Jack bully you: stand up … him.

Analyze the question: The sentence is "Don't let Jack bully you: stand up ____ him." We need the preposition that correctly completes the phrasal verb with "stand up."

Explain the concept: "Stand up" combines with different prepositions to give different meanings:

• stand up for someone = defend or support them (e.g., "Stand up for your sister.")
• stand up to someone = confront or resist them bravely, especially a bully or someone more powerful
• stand up against is usually used with a cause, system, or idea (e.g., "stand up against injustice"), not typically a person in this sense

Step-by-step solution:

1. The context is bullying: Jack is the one troubling you.
2. You are being told to resist or face Jack courageously, not to defend him.
3. "Stand up for him" (A) would mean you support or defend Jack, which contradicts the sentence.
4. "Stand up above him" (C) makes no sense.
5. "Stand up against him" (B) is understandable, but the standard idiomatic phrase for confronting a bully or an intimidating person is "stand up to."
6. So "stand up to him" is the best fit.

Answer: D. to

Tip: Remember the pair:

• stand up FOR = on someone's side (defend)
• stand up TO = face someone (confront)

If a bully is involved, think "stand up TO the bully."