Correct Answer - Option 3 : Ampere’s law
Biot-savart’s law states that the magnetic intensity at any point due to a steady current in an infinitely long straight wire is directly proportional to the current and inversely proportional to the distance from point to wire. It is a general modification of ampere’s law.
\(dH = \frac{{\vec I \cdot d\vec l \times \widehat {{i_r}}}}{{4\pi {R^2}}} = \frac{{\vec I \cdot d\vec l \times \vec R}}{{4\pi {{\left| {\vec R} \right|}^3}}}\)
\(H = \oint \frac{{\vec I \cdot d\vec l \times \widehat {{i_r}}}}{{4\pi {R^2}}}\)
Ampere's Law states that for any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop.