A

To determine the magnetic field of a toroid, we can use Ampère's law, which states that the line integral of the magnetic field around a closed loop is equal to the permeability times the current enclosed by that loop.
Step 1: For a toroid, we consider a circular path of radius r (the mean radius of the toroid) and we can express the magnetic field B inside the toroid as uniform.
Step 2: According to Ampère's law:
\[ B \cdot 2 \pi r = \mu_0 n I \]
where \( n \) is the number of turns per unit length and \( I \) is the current.
Step 3: Rearranging gives us the expression for the magnetic field B:
\[ B = \frac{\mu_0 n I}{2 \pi r} \]
Thus, Option A corresponds to this derived formula. Therefore, the correct answer is A.