The dimensional formula for the magnetic fields is
A
$$ [ML^2T^{-1}A^{-2} ] $$
Correct option is A. $$ [MT^{-2} A^{-1} ] $$
We have the equation,
$$F=qvB\,sin\theta$$
Dimension of Force, $$F=[MLT^{-2}]$$
Dimension of Velocity, $$v=[LT^{-1}]$$
Dimension of Charge, $$q=[AT]$$
$$Sin\theta $$ is dimensionless
Then,
Magnetic Field, $$B=\dfrac{Force}{Velocity \times Charge}=\dfrac{[MLT^{-2}]}{[LT^{-1}][AT]}=[MT^{-2}A^{-1}]$$