Isotopes used in proving semi-conservative replication of DNA were

The correct option is A ${}^{14}\text{N}{,}^{15}\text{N}$
Semiconservative DNA replication
In semi-conservative DNA replication each one of the newly synthesised DNA has one half of the parental DNA and one half of the new DNA.

Proof for DNA replication
The semi-conservative replication of DNA was proved by Messelson and Stahl by conducting experiments using $Escherichiacoli$. They performed the DNA replication experiment by incorporating a heavy isotope of nitrogen (${}^{15}\text{N}$) and a light isotope of nitrogen (${}^{14}\text{N}$) to the DNA of $Escherichiacoli$.

Meselson and Stahl’s experiment
Meselson and Stahl’s experiment is as follows:
$\bullet$They grow bacterium E.coli for many generations in a medium containing ${}^{15}\text{N}{H}_{4}Cl{.}^{15}\text{N}$ is the stable heavy isotope of nitrogen.
$\bullet$The ${}^{15}\text{N}$ labelled bacterial cells are then transferred to a medium containing normal nitrogen ${}^{14}\text{N}$.
$\bullet$After one generation, when bacterial cells had multiplied into two, they isolated the DNA and found that the density of the newly formed cell’s DNA is intermediate between ${}^{15}\text{N}$ and lighter ${}^{14}\text{N}$ DNA.
$\bullet$This was because during replication, new DNA molecules are formed with one ${}^{15}\text{N}$ old strand and ${}^{14}\text{N}$ new strand.
$\bullet$The daughter cells were allowed to divide again.
$\bullet$Out of the four second generation cells, DNA of two cells were found to have intermediate density and the remaining two cell’s DNA were with light density.
$\bullet$This proved that in replication one of the parent DNA is conserved in the daughter. While the second is freshly synthesised.

Pictorial representation of Meselson and Stahl’s experiment
Pictorial representation of Meselson and Stahl’s experiment:

Final answer
(C)${}^{14}\text{N}{,}^{15}\text{N}$