The process of reproductive cloning begins by
a. genetically modifying a stem cell.
b. placing an adult nucleus in a cell without a nucleus.
c. genetically modifying an embryo.
d. isolating cells to learn more about how they specialize

It would be B.
It depends on the cloning technique. The reproductive cloning procedure starts with extracting small samples of tissue from the subject that will be cloned. Then culture the tissues in a  lab. Condition the cultured cells so it halts cell division.  The G0 phase of the cell cycle then begins which selects a cell for such cloning, then step B comes into play

Gay-lussac's law i think.