The correct answer is $\boxed{\sqrt {\frac{{\text{M}}}{{\text{Q}}}}}$.
The effective depth of a slab is the distance from the top of the slab to the centroid of the tension reinforcement. The maximum bending moment of a simply supported slab is given by the following equation:
$$M = \frac{w l^2}{8}$$
where $w$ is the load per unit area, $l$ is the span of the slab, and $M$ is the maximum bending moment.
The M.R. factor is a factor that is used to account for the effects of shear and torsion on the bending moment of a slab. The M.R. factor is typically taken to be equal to 1.0 for slabs that are not subjected to significant shear or torsion.
The effective depth of a slab can be calculated using the following equation:
$$d = \sqrt {\frac{{\text{M}}}{{\text{Q}}}}$$
where $d$ is the effective depth of the slab, $M$ is the maximum bending moment, and $Q$ is the M.R. factor.
Therefore, the effective depth of a simply supported slab with a maximum bending moment of $M$ Kg.cm and an M.R. factor of $Q$ is given by the following equation:
$$d = \sqrt {\frac{{\text{M}}}{{\text{Q}}}}$$