Why are devices built for 802.11b and 802.11g compatible?

Devices built for 802.11b and 802.11g are compatible primarily because they use the same frequency band, specifically the 2.4 GHz band. This shared frequency allows devices adhering to either standard to communicate with each other since the radio signals can overlap.

Furthermore, both standards support the same modulation techniques, which means they can transmit and receive data effectively within that frequency range. Consequently, while they have different maximum data rates—802.11b supports up to 11 Mbps and 802.11g allows for speeds up to 54 Mbps—the frequency compatibility ensures that devices operating under these standards can still work together.

The other options may contain elements of truth but do not accurately capture the primary reason for their compatibility. For instance, while it's true they are both part of the 802.11 family of standards, this alone does not guarantee interoperability between devices, as differences in frequency and transmission methods could impede communication. The other choices also misrepresent aspects such as transmission capacity and networking protocols, which vary between the two standards.