This Buddypress plugin by Adam J Nowak adds a customizable security question to the sign up form. You can choose your own question and answers.
Multiple answers are possible, you can have a question like: Do you like cats or dogs and have both as acceptable answers. The goal is to fool the spam bots. Change questions as often as you need.
Plugin homepage: http://hyperspatial.com/wordpress-development/plugins/buddypress-humanity/