Explanation:

- A piece of cake is an idiom that means something is very easy to do.

- Option 1: A hidden enemy. This refers to an adversary that is not visible or is unexpected. It has no relation to the idiom "a piece of cake."

- Option 2: A dangerous situation. This describes a perilous or risky circumstance. It doesn't align with the meaning of the idiom.

- Option 3: A very easy task. This is the correct meaning of the idiom "a piece of cake." It's used to describe something that can be accomplished with little effort.

- Option 4: A thoughtful mind. This refers to someone who is contemplative or considerate, unrelated to the idiom.