Which of the following radio frequencies would have the least problem with dead spots?

Question

a) 100 MHz
b) 850 MHz
c) 1900 MHz
d) 2.1 GHz

Answer 1

I assume you mean dead spots in buildings.
I conclude that - the higher the frequency the higher the penetration ability of the wave (example LW radio = worst, X-ray = best).
I therefore vote for 2.1 GHz.

Answer 2

100 MHZ the longer the wavelength the less powerful transmission.