The correct answer is D.
OBJ-5.4: Wireless B/G networks utilize 2.4 GHz, while Wireless AC uses 5.0 GHz. Wireless N can support both 2.4 GHz and 5.0 GHz frequencies. The most likely cause of the issue is that the older access point supported both 2.4 GHz (for older devices) and 5.0 GHz (for newer devices). Since you installed a brand new 802.11ac access point, it is only broadcasting at 5.0 GHz and is preventing the older printer from connecting due to a frequency mismatch. Since the other devices are all connected to it without any issues, it is unlikely to be an issue with the transmission power or the password. With Wireless AC, the channel is automatically configured by the access point by default since there are 24 non-overlapping channels to choose from, making it an unlikely source of this issue.