It should be 3 or 6... Not 100% sure.
I believe the answer you are looking for has to do with cells, they are the basic building block of life and are the smallest life that can support itself.
He is correct, the correct answer is
B. club mosses, horsetails, and ferns.
Answer:
Semi-conservative
Explanation:
Meselson–Stahl experiment proved that DNA replication is a semiconservative process, feeding E. colo bacteria with different types of Nitrogen isotopes (N14 and N15).
Changing the type of radioactive compound in different times of bacteria grow, they could observe that both of the isotopes were present on the bacteria culture.
Basically, if the bacteria present both of them, the DNA was replicated semiconcervely.
Guess what? they show both! (See the image for more information)
Living
Genes
Cells or genes