Note: From $y = a\cos bt$, the frequency $f = b / 2\pi$

For $5 \cos (20\pi t): ~ f_1 = 20\pi / 2\pi = 10$

For $2 \cos (40\pi t): ~ f_2 = 40\pi / 2\pi = 20$

For $\cos (80\pi t): ~ f_3 = 80\pi / 2\pi = 40$

fundamental frequency, $f_0$ = lowest frequency

$f_0 = 10$ ← Answer: [ A ]