What is the proper format of a custom auto-attendant prompt?

What is the proper format of a custom auto-attendant prompt?

A.
G.711 u-law, 8 kHZ, 8-bit, mono

B.
G.711 a-law, 8 kHz, 16-bit, mono

C.
G.711 a-law, 16 kHZ, 8-bit, stereo

D.
G.729, 8 kHZ, 8-bit, mono