Can some one lifed me up and help find my love that i dont have

Me because some of question i don’t understand

Which of the following is most likely the next step in the series? a3z, b6y, c9x, d12w, е15v, f18u

Im gonna fail 7th grade selective breeding 1. creates offspring which are genetically identical to the parent 2. is the process of breeding only organisms with desirable traits 3. involves the removal of the nucleus of a cell 4. combines traits from organisms of different species