Consider the system as shown in the figure. The pulley and the string are light, and all the surfaces are frictionless. The tension in the string is $(g=10m/s^2)$.

The correct option is A $5N$
Draw FBD for mass $m_1$ and $m_2$.


Find the tension in the string.

From FBD of block $m_1$
$m_{1}g-T=m_{1}a\dots \left(i\right)$

From FBD of block $m_2$
$T=m_{2}a\dots \left(ii\right)$
By solving equation $\left(i\right)$ and $\left(ii\right)$,
we get tension,$T=5N$

Final answer: $\left(d\right)$