Answer: mitosis.
During DNA replication a pair of chromosomes of from one cell genders two duplicated chromosomes (so the number of chromosomes is duplicated), then in the mitosis process the cell split into two identical cells each with half of the duplicated chromosomes, so at the end one cell with two chromosomes form two cells each with two chromosomes. So, the new generation of cells has the same number of chromosomes.
I am going with x-rays; it is the most visible to the naked eye out of all the choices.
Skeletal muscle is under voluntary control
I think it's electric force. Not 100% sure but I hope it's right.