Why should you not set your transmit frequency to be exactly at the edge of an amateur band or sub-band?

Answer: D. All these choices are correct

All these choices are correct. Your frequency display may be slightly off, your signal's sidebands take up space on either side of the dial frequency, and transmitters can drift. FCC rules 97.101 and 97.301 require your whole signal to stay inside the band.

Tip: Stay a little inside band edges; your signal is wider than the dial number.

FCC rule reference: Part 97.101(a), 97.301(a-e)