Which of the following frequencies falls in the range of rf waves used by commercial radio broadcasting stations?

a. 6,000 hz
b. 6,000,000 hz
c. 60 hz
d. 600,000 hz

A).  6,000 Hz.
No.  This is in the middle of the human hearing range.  If a radio station were transmitting on this frequency, then every wire and cable in town would pick up a little of its energy, and you'd hear it on every telephone, earbud, and headphone, and it would come out of every loudspeaker.

B).  6,000,000 Hz.  (6 mHz)
No.  This isn't used by 'commercial' broadcasting stations, but it IS used by
foreign short-wave stations, to broadcast news, 'culture', and propaganda
into other countries. 

C).  60 Hz.
No.  This is the frequency of the AC that you use when you plug anything
into a wall socket.  All of the electric company's power lines RADIATE a
little bit of energy at this frequency, so it wouldn't be a very smart frequency
for a radio station to try to use, since the whole country is already blanketed
with interference at this frequency.

D).  600,000 Hz  (600 kHz)
Yes !  This is the "600" or "60" or "6" that's marked
near the left end of your AM radio dial.
yay !

Wavelength divides in half and speed stays the same.