Given below are two statements: one is labelled as Assertion (A)...

The time period of oscillation of a magnetic dipole in a uniform magnetic field is given by $T = 2\pi \sqrt{\dfrac{I}{MB}}$.

From this formula, it is clear that $T \propto \dfrac{1}{\sqrt{B}}$. Hence, the Reason (R) is true.

If the magnetic field is increased by $300\%$, the new magnetic field becomes:
$B' = B + \dfrac{300}{100}B = 4B$

The new time period $T'$ will be:
$T' \propto \dfrac{1}{\sqrt{B'}} = \dfrac{1}{\sqrt{4B}} = \dfrac{1}{2\sqrt{B}}$

Therefore, $T' = \dfrac{T}{2}$. The time period becomes half of its initial value. Hence, the Assertion (A) is true.

Since the inverse square root relationship directly leads to the halving of the time period, the Reason (R) is the correct explanation of the Assertion (A).

Answer: Both (A) and (R) are true and (R) is the correct explanation of (A).