The correct answer is $\boxed{\text{C}}$.
The output signal has the same frequency as the input signal, but its amplitude is attenuated by the filter. The filter attenuates frequencies above the cutoff frequency, so the output signal will have a frequency of 1 kHz, but its amplitude will be lower than the amplitude of the input signal.
The other options are incorrect because they do not represent the frequency of the output signal. Option A is incorrect because the output signal has a frequency of 1 kHz, not 0 Hz. Option B is incorrect because the output signal has a frequency of 1 kHz, not 0.75 kHz. Option D is incorrect because the output signal has a frequency of 1 kHz, not 0.25 kHz.
Here is a more detailed explanation of the answer:
The sampling theorem states that a continuous-time signal can be perfectly reconstructed from its samples if the sampling rate is greater than twice the highest frequency component of the signal. In this case, the input signal has a frequency of 1 kHz, so the sampling rate must be greater than 2 kHz. The sampling rate in this case is 1500 samples/sec, which is greater than twice the highest frequency component of the signal, so the input signal can be perfectly reconstructed from its samples.
The low-pass filter attenuates frequencies above the cutoff frequency. In this case, the cutoff frequency is 800 Hz, so the filter will attenuate frequencies above 800 Hz. The output signal will therefore have a frequency of 1 kHz, but its amplitude will be lower than the amplitude of the input signal.