What sets the minimum detectable signal level for a direct-sampling software defined receiver in the absence of atmospheric or thermal noise?

Answer: B. Reference voltage level and sample width in bits

The smallest signal an ADC can resolve is one step, which equals the reference voltage divided by 2 to the number of bits. So the reference voltage and the sample width set the minimum detectable level.

Tip: Smallest step = Vref divided by 2^bits.