The problem describes a signal with two baseband components: one has a rectangular spectrum, and the other has a triangular spectrum, modulated onto two different carriers. The spectrum $Y(f)$ of the received signal shows these two components centered at different frequencies, ensuring that their spectra do not overlap.

From the image:

1. The rectangular spectrum is centered at 110 kHz and spans from 85 kHz to 135 kHz (a bandwidth of 50 kHz).
2. The triangular spectrum is centered at -110 kHz and spans from -135 kHz to -85 kHz.

The signal is demodulated by mixing with a cosine at 110 kHz. This shifts the rectangular baseband signal centered at 110 kHz down to 0 kHz (baseband). The triangular spectrum, originally centered at -110 kHz, is shifted by the same 110 kHz, meaning it will now appear centered at -220 kHz.

To isolate the rectangular spectrum (now at baseband), an ideal low-pass filter must pass the signal from 0 kHz to 25 kHz (the positive half of the baseband signal), since the rectangular spectrum originally spanned from 85 kHz to 135 kHz.

Therefore, the correct answer is:

A. 25 kHz

Would you like further details or have any other questions?

Here are 5 related questions to expand your understanding:

1. What is the significance of the triangular and rectangular spectrum shapes in real-world signals?
2. How does demodulating a signal with a cosine shift the spectrum?
3. What would happen if the triangular signal had a frequency closer to 110 kHz?
4. Why is it important that the signals occupy non-overlapping regions of the spectrum?
5. How does a low-pass filter affect signals after demodulation?

Tip: When using a cosine for demodulation, the frequency of the cosine defines the shift, and the result needs careful filtering to isolate the desired component.