The correct answer is $\boxed{\text{B) 0.3}}$.
The maximum bending moment in a simply supported slab spanning in two directions is given by the following equation:
$$M = \frac{w l^2}{8}$$
where $w$ is the uniform load per unit area, $l$ is the length of the slab, and $b$ is the width of the slab.
If the sides of the slab are equal, then the length and width of the slab are equal, and the maximum bending moment is given by:
$$M = \frac{w l^2}{8} = \frac{w b^2}{8}$$
The maximum bending moment is multiplied by 0.3 when the sides of the slab are equal.
Option A is incorrect because the maximum bending moment is not multiplied by 0.2.
Option B is correct because the maximum bending moment is multiplied by 0.3.
Option C is incorrect because the maximum bending moment is not multiplied by 0.4.
Option D is incorrect because the maximum bending moment is not multiplied by 0.5.