In the figure shown, all the blocks are of equal mass of $10\text{kg}$ and $F=300\text{N}$. Find the tension in the string $T_{AB}$. Assume all surface to be smooth and $g=10{\text{m/s}}^2$.

The correct option is A $100\text{N}$
Considering all the blocks to be a single system ie. $(10+10+10=30kg)$, FBD of the system will be

$F-30g\sin 30°=30a$
$\Rightarrow 300-150=30a$
$\Rightarrow a=5{\text{m/s}}^2$
Hence whole system will be accelerating upwards at $5{\text{m/s}}^2$
Considering FBD of block A,

$T_{AB}-10g\sin 30°=10a$
$\Rightarrow T_{AB}=50+50=100\text{N}$