Which of the following was used by Meselson and Stahl to separate DNA during their experiment on the semi-conservative nature of DNA replication?

The correct option is C Cesium chloride
Meselson and Stahl utilised the technique of density gradient centrifugation in their experiments to prove semi-conservative nature of DNA replication. They grew E.coli in a medium containing ${}^{15}$NH${}_4$Cl. Consequently, all the DNA molecules got ${}^{15}$N, the heavier isotope of nitrogen, incorporated into them. Following that, the cells were transferred into a medium containing normal NH${}_4$Cl (ammonium chloride). The cells were isolated at different time intervals, DNA was extracted and separated in CsCl density gradients.

Density gradient centrifugation is a technique that helps in the separation of substances based on density, size, shape, viscosity of the medium.

Meselson and Stahl used cesium chloride as the medium due to the following advantages:
Cesium, being a heavy element, formed a denser salt solution, appropriate for the DNA to float before centrifugation.
Cesium salts are mild enough to prevent the bacterial DNA from further disintegration.
On ultracentrifugation, a density gradient was formed by cesium chloride spontaneously which was appropriate for DNA separation.

When the isolated DNA was ultracentrifuged in the CsCl density gradient, the molecules with different densities started separating and forming discrete bands in the centrifugation tube. This helped to conclude the type of nitrogen incorporated in it and hence predict the nature of DNA replication.

Protease are enzymes which breakdown proteins. Nucleases are enzymes that break down nucleic acids. These were used by Avery, MacLeod, McCarty in their experimental setup to prove that DNA is the genetic material.