$(a)$ What is meant by a 'magnetic field'?
$(b)$ How is the direction of magnetic field at a point determined?
$(c)$ Describe an activity to demonstrate the direction of the magnetic field generated around a current-carrying conductor.
$(d)$ What is the direction of the magnetic field at the centre of a current-carrying circular loop?

(N/A) magnetic field is the region surrounding a magnet or a current-carrying conductor in which its magnetic influence can be experienced by other magnetic materials.
$(b)$ The direction of the magnetic field at a point is determined using a magnetic compass. The north pole of the compass needle points in the direction of the magnetic field. It can also be determined using the Right-Hand Thumb Rule.
$(c)$ Activity: Take a piece of cardboard and pass a straight copper wire through its centre,perpendicular to the plane of the cardboard. Connect the ends of the wire to a battery and a switch. Sprinkle some iron filings uniformly on the cardboard. When the current is passed through the wire,tap the cardboard gently. You will observe that the iron filings align themselves in a pattern of concentric circles around the wire. These circles represent the magnetic field lines. The direction of these lines can be determined by placing a magnetic compass at different points on the circles. If the direction of the current is reversed,the direction of the magnetic field lines also reverses.
$(d)$ At the centre of a current-carrying circular loop,the magnetic field lines are straight and perpendicular to the plane of the loop.