Which security control is most in protecting against eavesdropping on wireless lan (wlan) data transmissions that would jeopardize confidentiality?

SSL encryption

It is easy for eavesdroppers to capture passwords, logins and sensitive information on public networks. 

VPN (Virtual Private Networks) is one way to provide encryption before eavesdroppers can grab packets of the network traffic.

Another way that privacy is provided is to use SSL encryption. On websites such as banking sites that provide encryption, the web address begins with https:// instead of http://.