An electromagnet is a coil (solenoid) of wire wound around a core; passing current through the coil produces a magnetic field, and the core intensifies it. The magnetic field produced can be summarised by the relation B ∝ μ × N × I / L, where μ is the permeability of the core material, N is the number of turns, I is the current, and L is the length of the coil. To make the field as strong as possible, we want to maximise the permeability, the number of turns and the current.

Choosing the right combination:

1. Core material — soft iron: Soft iron has very high magnetic permeability, so it concentrates the field lines strongly. Crucially it also has low retentivity, meaning it loses almost all its magnetism the instant the current is switched off. That is essential for a scrap-lifting magnet, which must grab metal when energised and drop it when de-energised.
2. More turns (N): Increasing the number of coil turns increases the field in direct proportion.
3. More current (I): Increasing the current likewise increases the field.

Therefore the combination of a soft iron core, increased turns, and increased current gives the strongest field — matching the correct option.

Why the other combinations fail: The choice using a steel core with fewer turns and lower current is doubly weak — steel has lower permeability and high retentivity (it would stay magnetised and refuse to release scrap), and reducing turns and current lowers the field. The copper core with lower current option is wrong because copper is essentially non-magnetic (it does not concentrate field lines at all), and lower current weakens the field. The air core with fewer turns gives the weakest field of all, since air has the lowest permeability and the reduced turns cut the field further.