If resistance is 20 ohms. and inductance is 27 in a R-L series circuit, then time constant of this circuit will be

The correct answer is A. 0.001 s.

The time constant of an R-L series circuit is given by the formula:

$$\tau = \frac{L}{R}$$

where $L$ is the inductance in Henrys and $R$ is the resistance in Ohms.

In this case, we are given that $L = 27\text{ H}$ and $R = 20\text{ Ω}$. Substituting these values into the formula, we get:

$$\tau = \frac{27\text{ H}}{20\text{ Ω}} = 0.001\text{ s}$$

Therefore, the time constant of this circuit is 0.001 s.

Option B is incorrect because it is 10 times greater than the actual value. Option C is incorrect because it is 100 times greater than the actual value. Option D is incorrect because it is 1000 times greater than the actual value.