You have taken E coli strain A which has its own plasmids and mixed it with E coli B strain without plasmids E coli B cells now have plasmids but in addition they also carry some genes from E coli A strain. What has happened is:.
A. The E coli B cells have become mutants of the original culture.
B. Vertical transmission of DNA has occurred.
C. An example of transformation.
D. The calls of E coli A were Hfr cells.

Explanation:

1. Hfr strain;  A strain of bacterial that possesses the F factor which is  integrated into the their genome, and when it conjugates with another bacterium, it may transfer a copy of the F factor as well as a small part or the entire chromosome( genome) to the recipient bacterium.

2. Hfr cells are able to convert F- recipient ( which lacks F plasmid) cells to F+.

3. As given here, E. coli strain A has its own plasimd and when mixed with plasmid B ( which do not have its own plasmid), then the plasmid B gain plasmid as well as some genes from strain A, it indicate that the cells of E. coli A were Hfr cells.