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 has the ability to support both 2.4 GHz and 5.0 GHz frequencies. The most likely cause of the issue is that the older WAP supported 2.4 GHz (for older devices) and 5.0 GHz (for newer devices). Since you installed a brand new 802.11 AC WAP, it is only broadcasting at 5.0 GHz, which prevents the older printer from connecting due to a frequency mismatch.