Concept: For a square root to be defined, the expression inside it must be non-negative.Explanation: The function is f(x)=x−1+3−x.For x−1 to be real, we need x−1≥0.So, x≥1.For 3−x to be real, we need 3−x≥0.So, x≤3.Both conditions must hold together.Therefore, 1≤x≤3.This gives the domain as the closed interval [1,3].Answer: Option C: [1,3]