The magnetic field at a distance $r$ from a LONG STRAIGHT wire carrying current $I$ is

1. Apply Ampere's law with a circular Amperian loop of radius $r$ centred on the wire. 2. By symmetry, $\vec{B}$ is tangential to the loop with the same magnitude everywhere on it. 3. $\oint\vec{B}\cdot d\vec{l} = B\,(2\pi r) = \mu_0 I$. 4. Solve: $B = \dfrac{\mu_0\,I}{2\pi r}$. 5. Field circles around the wire (right-hand rule: thumb along $I$, fingers curl in the direction of $\vec{B}$). 6. Option B drops a factor of 2. Option C has wrong distance dependence. Option D would grow with $r$. _Source: syllabus Physics Part 1, Ch 4, §4.5 (Ampere's Law applied to a long straight wire), p. 15–16._