We cannot take the square root of a negative number without using complex numbers. Because of this, both 5x and x+3 must be non-negative in order for the product to be defined.
5x ≥ 0 implies that x ≥ 0
x + 3 ≥ 0 implies that x ≥ -3
Both of these are satisfied if and only if x ≥ 0 (note that if x ≥ 0, then x is automatically greater than or equal to -3). Therefore, the correct answer is x ≥ 0.
