What is the difference on all the Wireless Standards including 802.11a, 80211b/g/n, & 802.11ac

Which one to choose and is best for WiFi wireless penetration testing?

The explanation of 802.11 WiFi Family Standards for Penetration Testing:

Frequency
IEEE 802.11bgn = 2.4GHz only
IEEE 802.11gn = 2.4GHz only
IEEE 802.11agn = 2.4GHz + 5GHz
IEEE 802.11ac = 2.4GHz + 5GHz
abgn