Which of the following is the mean number per cross of F2 generation offspring that are the result of crossing over

2.2

Explanation:

Since all of the mice in the F1 generation are heterozygous for both traits, they contain a copy of each on separate chromosomes. (AB on one from parent 1 and ab on the other from the parent 2.)  If these chromosomes split away from each other without crossing over, the F2 offspring can ONLY by AABB or aabb.  This means that any heterozygous combinations are the result of crossing over and must be counted.  When all are counted, we get a total of 11, or 2.2 per cross.