The correct answer is $\boxed{\text{B. }5.0 \text{ cm}}$.
The center of gravity of a T section is located at a distance of $5.0 \text{ cm}$ from its bottom. This can be calculated by using the following formula:
$$d = \frac{bh}{2b + h}$$
where $d$ is the distance from the bottom of the T section to the center of gravity, $b$ is the width of the flange, $h$ is the height of the web, and $2b + h$ is the total depth of the T section.
In this case, $b = 10 \text{ cm}$, $h = 5 \text{ cm}$, and $2b + h = 20 \text{ cm}$. Substituting these values into the formula, we get:
$$d = \frac{10 \times 5}{2 \times 10 + 5} = 5.0 \text{ cm}$$
Therefore, the center of gravity of a 10 Ã 15 Ã 5 cm T section is located at a distance of $5.0 \text{ cm}$ from its bottom.
Option A is incorrect because it is the distance from the top of the T section to the center of gravity. Option C is incorrect because it is the distance from the center of the T section to the bottom. Option D is incorrect because it is the distance from the center of the T section to the top.