The idiomatic expression in the text is:

2. have very little in common

The reason option 2, "have very little in common," is the use of an idiomatic expression is that it conveys a specific meaning that goes beyond the literal interpretation of the individual words. It implies that two people do not share many similarities or interests, and it's a common phrase used to describe differences in relationships. The other options do not contain idiomatic expressions and can be understood literally.