Two resistance are connected in series as shown in diagram. What is the value of $R$?

Voltage across $5\Omega$ resistor is $V=10V$

Current, $I=\dfrac{V}{R}=\dfrac{10}{5}$

$\implies I=2A$

Since resistance $5\Omega$ and $R$ are in series and hence current through both resistance is same.

Hence current through $R$ is also $2A$.

Now for $R$, potential difference is $V=6V$ and $I=2A$.

Hence , $R=\dfrac{V}{I}=\dfrac{6}{2}$

$\implies R=3\Omega$

Answer-(D)