The Biot-Savart law gives the magnetic field $d\vec{B}$ due to a small current element $I\,d\vec{l}$ at a point $\vec{r}$ from the element. Its formula is

1. NCERT §4.3 states the Biot-Savart law in vector form: $d\vec{B} = \dfrac{\mu_0}{4\pi}\,\dfrac{I\,d\vec{l}\times\hat{r}}{r^2}$. 2. KEY FEATURES: - Inverse-SQUARE dependence on distance — like Coulomb's law for electric fields. - Direction given by the CROSS product $d\vec{l}\times\hat{r}$. - Magnitude: $dB = \dfrac{\mu_0\,I\,dl\,\sin\theta}{4\pi r^2}$, where $\theta$ is the angle between $d\vec{l}$ and $\hat{r}$. 3. Constants: $\mu_0/(4\pi) = 10^{-7}\,\text{T\,m/A}$ exactly (in SI). 4. Options B, C, D have wrong distance dependence or missing cross product. _Source: NCERT Class 12 Physics Part 1, Ch 4, §4.3 (Biot-Savart Law, Eq. 4.8), p. 7–8._