The correct answer is $\boxed{\text{A. 310 Ohm}}$.
The zener voltage is the voltage that the zener diode will regulate the output voltage to. In this case, the zener voltage is 6.8 V. The load current is the current that the load will draw from the output of the zener regulator. In this
case, the load current is 5 to 20 mA.The series resistor is used to limit the current that flows through the zener diode. The value of the series resistor is calculated using the following formula:
$$R = \dfrac{(V_{IN} – V_{Z})}{I}$$
where:
- $R$ is the value of the series resistor in ohms
- $V_{IN}$ is the input voltage in volts
- $V_{Z}$ is the zener voltage in volts
- $I$ is the load current in amps
In this case, the input voltage is 15 to 20 V, the zener voltage is 6.8 V, and the load current is 5 to 20 mA. Substituting these values into the formula, we get:
$$R = \dfrac{(15 V – 6.8 V)}{5 mA} = 310 \Omega$$
Therefore, the value of the series resistor should be 310 ohms.
Option A is the correct answer because it is the only option that is within the range
of values that are typically used for zener regulators. Option B is too high, option C is too low, option D is too high, and option E is not a valid option.