Carrier frequency in FM equation

What is the value of carrier frequency in the following equation for the FM signal ?
v(t)= 5 cos(6600t+ 12sin2500t)

Options:

a) 1150 Hz
b) 6600 Hz
c) 2500 Hz
d) 1050 Hz

Correct Answer: d) 1050 Hz

Explanation:

A standard FM signal is represented by
v(t)= A_c cos(2πf_ct+ k_fsin2πf_mt)
A_c = carrier amplitude
f_c= carrier frequency
k_f= modulation index
f_m = modulating frequency
k_f = frequency deviation/modulating frequency
therefore, f_c = 6600/2π
= 1050Hz