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

The correct answer is: on.

The preposition “on” is used to indicate that someone or something can be depended on. In this case, the speaker is saying that they can depend on the person to be there whenever they need them.

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
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  do the right thing.
• I can trust you to keep your word.