To determine the concentration of $ \mathrm{B} $ when the rate of formation of $ \mathrm{B} $ is set to zero, let's consider the reaction sequence:

$$ \mathrm{A} \xrightarrow{\mathrm{K}_1} \mathrm{B} \xrightarrow{\mathrm{K}_2} \mathrm{C} $$

The rate of formation of $ \mathrm{B} $ from $ \mathrm{A} $ is given by:

$$ \text{Rate of formation of } \mathrm{B} = \mathrm{K}_1[\mathrm{A}] $$

The rate of consumption of $ \mathrm{B} $ to form $ \mathrm{C} $ is given by:

$$ \text{Rate of consumption of } \mathrm{B} = \mathrm{K}_2[\mathrm{B}] $$

At steady state, where the rate of formation of $ \mathrm{B} $ is set to zero, the rate of formation of $ \mathrm{B} $ is equal to the rate of its consumption. Therefore, we have:

$$ \mathrm{K}_1[\mathrm{A}] = \mathrm{K}_2[\mathrm{B}] $$

Solving for $ [\mathrm{B}] $, we get:

$$ [\mathrm{B}] = \frac{\mathrm{K}_1}{\mathrm{K}_2} [\mathrm{A}] $$

Therefore, the correct concentration of $ \mathrm{B} $ is:

Option D: $$ \left(\frac{\mathrm{K}_1}{\mathrm{K}_2}\right)[\mathrm{A}] $$