1. **Problem statement:** We need to find the phase shift of the output signal when a sinusoidal input with frequency $f = \frac{1}{2\pi RC}$ is applied to an RC op-amp circuit. 2. **Formula and background:** For an RC circuit, the phase shift $\phi$ between input and output is given by: $$\phi = -\arctan(2\pi f RC)$$ where $f$ is the frequency of the input signal. 3. **Substitute the given frequency:** Given $f = \frac{1}{2\pi RC}$, substitute into the formula: $$\phi = -\arctan\left(2\pi \times \frac{1}{2\pi RC} \times RC\right)$$ 4. **Simplify the expression inside arctan:** $$2\pi \times \frac{1}{2\pi RC} \times RC = \cancel{2\pi} \times \frac{1}{\cancel{2\pi} RC} \times RC = 1$$ 5. **Calculate the phase shift:** $$\phi = -\arctan(1) = -45^\circ$$ 6. **Interpretation:** The negative sign indicates the output lags the input by $45^\circ$. **Final answer:** The output signal is phase-shifted by $45^\circ$. Therefore, the correct option is **b) 45°**.