Final answer:
The logical NOT operator, represented by the symbol !, reverses the truth or falsehood of its operand. The correct answer to the student's question is option d. !, which is critical in programming to control logic flow.
Step-by-step explanation:
The operator in question that takes an operand and reverses its truth or falsehood is known as the logical NOT operator. In most programming languages, this is represented by the symbol !. So, the correct answer to the question is option d. !. For example, if we have a boolean variable isTrue, and it holds the value true, applying the ! operator like so !isTrue would result in false. Conversely, if isTrue was false, !isTrue would evaluate to true. This operator is fundamental in programming for controlling the flow of logic in conditional statements and loops.