The correct answer is C. distance.
The phrase “keep someone at arm’s length” means to avoid getting too close to them, either physically or emotionally. This is often done when someone is suspicious of the other person’s motives or intentions.
The other options are not correct because they do not make sense in the context of the phrase.
A. width: This would mean to avoid getting too close to someone physically, but it would not necessarily mean to avoid them emotionally.
B. length: This would mean to avoid getting too close to someone physically or emotionally, but it is not a common way of expressing this idea.
D. aim: This would mean to avoid trying to hit someone with your arm, but it does not make sense in the context of the phrase.