The condition must be a . . . . . . . . expression.

The correct answer is C. Boolean.

A Boolean expression is an expression that evaluates to either true or false. It is used in conditional statements, such as if statements and while statements.

An integer is a whole number, such as 1, 2, 3, and so on.

A double is a floating-point number, such as 1.23, 3.14, and so on.

A short is a signed integer that can store values from -32768 to 32767.

In the context of the question, the condition must be an expression that evaluates to either true or false. Therefore, the correct answer is C. Boolean.