Which one of the following expressions represents a frequency modulated signal?

A. $${A_c}\cos 2\pi \left[ {{f_c}t + m\left( t \right)} \right]$$

B. $$\left[ {1 + \int\limits_0^t {m\left( \tau \right)d\tau } } \right]\cos 2\pi {f_c}t$$

C. $${A_c}m\left( t \right)\cos 2\pi {f_c}t$$

D. $${A_c}\cos 2\pi \left[ {{f_c}t + \int\limits_0^t {m\left( \tau \right)d\tau } } \right]$$

Answer: Option D