The correct answer is: Only 800 Hz component.
A 1.0 kHz signal is flat-top sampled at the rate of 1800 samples/sec. This means that the sampling frequency is 1800/1000 = 1.8 times the fundamental frequency of the signal. This is a Nyquist sampling rate, which means that the signal can be perfectly reconstructed from the samples.
The samples are applied to an ideal rectangular LPF with cut-off frequency of 1100 Hz. This means that the filter will pass all frequencies below 1100 Hz and attenuate all frequencies above 1100 Hz.
The output of the filter will therefore contain only the frequencies that are present in the input signal and that are below the cut-off frequency of the filter. In this case, the only frequency that meets these criteria is 800 Hz. Therefore, the output of the filter will contain only an 800 Hz component.
The other options are incorrect because they include frequencies that are above the cut-off frequency of the filter. These frequencies will be attenuated by the filter and will not be present in the output.