You can rely . . . . . . . . him to be there whenever you need him. A. with B. at C. in D. on

The other options are not as appropriate in this context. “With” is used to indicate that someone or something is associated with or connected to something else. “At” is used to indicate a location or position. “In” is used to indicate a state or condition.

Here are some examples of how the preposition “on” can be used:

• You can rely on me to help you.
• I can count on you to be there for me.
• I can depend on you to do the right thing.
• I can trust you to keep your word.