The expression "to smell a rat" is an idiomatic phrase, and it does not refer to actually detecting the odor of a rat. Instead, it is used figuratively to indicate suspicion or a sense that something is not quite right in a situation. When someone "smells a rat," they believe that there may be deception, trickery, or dishonesty involved.

So, the correct explanation is:

Option 1, "To spot rats in a dark room," is a literal interpretation and does not capture the idiomatic meaning of the expression.

Option 2, "To misunderstand a speaker," is not related to the concept of suspicion or deceit.

Option 3: "To suspect a trick or a deceit."

Option 4, "To get confused," also doesn't capture the sense of suspicion or mistrust implied by the expression.