What product is formed in the Wurtz reaction of 1-bromo-3-chlorocyclobutane?
Br is a much better leaving group than Cl.First of all presuming that Cyclobutane is stabilized by substitutent groups is important since it wil gradually rearrange itself to buta diene.But since it is a theoretical question hence  we can say that it should add to the Br side since Br leaves more efficiently being larger in size than Cl and hence having less efficient orbital overlap with Carbon as well as a large electronic repulsion due to the large number of electrons it has is the reason behind this.And the major product will probably be a bicyclo [1.1.0] butane since it is the kinetic product.
so basically your answer-