The perimeter of a rectangle, whose length is $10 \mathrm{cm}$ and breadth is $15 \mathrm{cm}$ will be:

Given:

Length of rectangle $\left(l\right)$ $=10 \mathrm{cm}$

Breadth of rectangle $\left(b\right)$ $=15 \mathrm{cm}$

Perimeter of rectangle $=2\times \left(l+b\right)$

So, the perimeter of the given rectangle will be

$$\begin{gathered} =2\times \left(10+15\right) \\ =2\times 25 \\ =50 \mathrm{cm} \end{gathered}$$

Hence, option B is correct.