Which of the following facts regarding the densities of 5d-series elements is correct?

The densities of 5d-series elements initially increase followed by a decrease.

A material substance's density is defined as its mass per unit volume. D is density, M is mass, and V is volume, therefore the formula for density is d = M/V. In terms of grams per cubic centimeter, density is frequently expressed.

Hence, option C is correct.