To find the output voltage \( V_o \) of the series regulator circuit, we need to analyze the voltage divider formed by the resistors and the operational amplifier settings.

The key to solving this problem is understanding how the feedback loop controls the output voltage. The operational amplifier (op-amp) is configured to maintain its inverting input at the same voltage as its non-inverting input.

Given that the non-inverting input is connected to a 6 V DC source, the inverting input of the op-amp will also try to maintain 6 V.

Using the voltage divider rule:

\(V_-=V_o\left(\frac{2 \, \text{k}\Omega}{2 \, \text{k}\Omega + 8 \, \text{k}\Omega}\right)\)

The inverting input voltage \( V_- \) can be set equal to the non-inverting input voltage of 6 V:

\(6=\frac{V_o \times 2 \, \text{k}\Omega}{10 \, \text{k}\Omega}\)

Solving for \( V_o \):

\(6=\frac{V_o}{5}\) \(V_o=30 \, \text{V}\)

Therefore, the value of \( V_o \) is 30 V.

Conclusion: The correct answer is 30 V.