Answer
C
Work Step by Step
Comparing $y=3\cos x$ with the standard form of the cos function $y=a\cos bx$, we find that $a=3$ while $b=1$. This means that the amplitude is 3 while the period is $\frac{2\pi}{b}=\frac{2\pi}{1}=2\pi$.
Therefore, we need to find a sin graph whose amplitude is 3 and whose period is $2\pi$. Such a graph is found in option C.