Introduction:
An overhead electric transmission line carrying a direct current generates a magnetic field around it. This magnetic field can be calculated using the right-hand rule for current-carrying conductors.

Given information:
- Current (I) = 2000A
- Distance below the line (d) = 4m

Calculating the magnetic field:
The magnetic field generated by a current-carrying conductor can be calculated using Ampere's Law or the Biot-Savart Law. In this case, we will use the Biot-Savart Law to determine the magnetic field at a distance below the line.

The Biot-Savart Law states that the magnetic field (B) at a point due to a current-carrying conductor is directly proportional to the current and inversely proportional to the distance from the conductor.

Formula:
B = (μ₀ * I) / (2π * d)

Where:
B is the magnetic field
μ₀ is the permeability of free space (4π × 10⁻⁷ T·m/A)
I is the current
d is the distance from the conductor

Calculating the magnetic field:
Substituting the given values into the formula, we can calculate the magnetic field generated by the overhead transmission line.

B = (4π × 10⁻⁷ T·m/A * 2000A) / (2π * 4m)

Simplifying the expression:

B = (8π² × 10⁻⁷ T·m) / (8πm)

The π and 8π in the numerator and denominator cancel out, leaving:

B = 10⁻⁷ T

Explanation:
The magnetic field produced by the overhead electric transmission line at a distance of 4m below it is 10⁻⁷ Tesla. The direction of the magnetic field can be determined using the right-hand rule, where the thumb points in the direction of the current and the curled fingers represent the magnetic field lines around the conductor.

Summary:
- An overhead electric transmission line carrying a direct current of 2000A generates a magnetic field.
- The magnetic field at a distance of 4m below the line is calculated using the Biot-Savart Law.
- The magnetic field is determined to be 10⁻⁷ Tesla.
- The direction of the magnetic field can be determined using the right-hand rule.

Option B is the ans.... ?...