What a lovely question. The estimate is 10-100 photons/bit (minimum).
If you’re curious about how many bits a single photon can carry, in controlled settings (tabletop quantum optics) a single photon can carry log(n) bits where n is the size of the state space of the photon, which theoretically is infinite and in practice it can reach into the hundreds/thousands.
No, the estimate is around 750 or 200 photons/bit received, depending on the transmission frequency. The answer to the question is B, not C. Your numbers are the estimated minimum needed, not the actual amount received, which is what the question was asking.
Visible light is different, because each photon has a lot more energy than in the 2.3GHz range. Your average decent consumer-level camera has a sensor that can nominally just about detect single photons some proportion of the time (as in, some of them bounce off instead of being detected) though it can't technically count them. The graininess on digital camera images is more from the Poisson noise of the incoming photons than it is from the applied noise of the sensor itself.
If you’re curious about how many bits a single photon can carry, in controlled settings (tabletop quantum optics) a single photon can carry log(n) bits where n is the size of the state space of the photon, which theoretically is infinite and in practice it can reach into the hundreds/thousands.