What is the real domain of the function \(f(x)=\sqrt{12-3x}\)?

Correct answer: \(( -\infty,4]\)

For a real square root, the expression inside the root must be non-negative. Thus, \(12-3x\geq 0\). Solving gives \(-3x\geq -12\), hence \(x\leq 4\). Therefore, the real domain is \(( -\infty,4]\). Option C incorrectly excludes 4, although \(f(4)=\sqrt{0}=0\) is defined. Exam tip: for real square-root functions, set the radicand greater than or equal to zero.