_______ is an equation describing the magnetic field generated by a constant electric current.

Correct option is B

Biot-Savart's Law is an equation that describes the magnetic field generated by a constant electric current. It provides a way to calculate the magnetic field at any point in space due to a small segment of current-carrying conductor.

​$$\mathbf{B} = \frac{\mu_0}{4\pi} \int \frac{I \, d\mathbf{l} \times \hat{r}}{r^2}\\\text{Where } \mathbf{B} \text{ is the magnetic field, } I \text{ is the current, } d\mathbf{l} \text{ is the infinitesimal length of the current element, } \hat{r} \text{ is the unit vector from the current element to the observation point, and } r \text{ is the distance between them.}$$​​