**Concept: The question tests on Functions **

**Solution:**

The function will be undefined if we get the square root of a negative number.

x^2−5x+6=x^2−2x−3x+6=x(x−2)−3(x−2)=(x−2)(x−3)

The roots of this quadratic equation is 2 and 3.

In any upward parabola, Any values within the parabola formed will be negative, i.e 2 < x < 3 and any values outside the parabola will be positive, i.e x < 2 and x > 3.

For eg if x = 2.5, then f(2.5) = (2.5 - 2) (2.5 - 3) = 0.5 * -0.5 = -0.25

Take other values outside the parabola. i.e x < 2 and x > 3

x < 2, eg if x = 1, then f(1) = (1 - 2) (1 - 3) = -1 * -2 = +2

x > 3, eg if x = 4, then f(4) = (4 - 2) (4 - 3) = 2 * 1 = +1

So for 2 < x < 3, the quadratic equation will be negative and the function is undefined.

Option B