Final answer:
OPTION A.The calculation of a magnetic field in an infinite solenoid is done using Ampere's Law, which relates the magnetic field to the electric current in the solenoid.
Step-by-step explanation:
The calculation of the magnetic field in an infinite solenoid typically involves using Ampere's Law. Ampere's Law states the magnetic field around an electric current is proportional to the current. It forms part of Maxwell's equations, that explain all electromagnetic phenomena.
When applying Ampere's Law to an infinite solenoid, we consider a closed loop or Amperian loop that only the parts of the loop that are inside the solenoid contribute to the line integral as the magnetic field outside is considered to be zero. The solenoid has 'n' turns per unit length, with the total enclosed current being the product of 'n', the current 'I', and the length 'l'. The B-field inside the solenoid is uniform and determined using Ampere's Law, which simplifies the integration process across the loop, offering an elegant solution to calculate the magnetic field.