What Evidence Supports the Contention That Other Sun-Like Stars Also

What evidence supports the contention that other sun-like stars also show a spot cycle much like our sun's sunspot cycle?

A) the variation in the rotation rates of sun-like stars
B) the variation in the nuclear reaction rates in sun-like stars
C) the variation in the strength of the H and K emissions in the spectra of sun-like stars
D) the variation in the parallax of sun-like stars
E) the variation in the color of sun-like stars