Chapter 4 - Graphs of the Circular Functions - Section 4.1 Graphs of the Sine and Cosine Functions - 4.1 Exercises - Page 143: 10

B

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