Suppose the two sinusoidal curves shown above are added together. which of the plots shown below best represents

The sinusoidal curves are shown in the picture attached.

Note that the two waves have the same amplitude (same "height"), the same wavelength (same distance from one peak to the next one) and therefore the same frequency, but different phase (they start in different points).

When you sum two different sinusoidal curves with the same amplitude and frequency, but different phase you get a sinusoidal wave with the same frequency as the original curves, but a different amplitude and a different phase.

Therefore, the correct answer is graph B).