Identify the force and perpendicular distance from the axle.

Force $F = 5.0 \, \text{N}$
Perpendicular distance $d = 8.0 \, \text{cm} = 0.08 \, \text{m}$

The moment about the axle produced by the force on the right-hand side is $\boxed{0.40 \, \text{Nm}}$.

a) (ii) Write down the moment about the axle produced by the force on the left hand side of the coil.
The force on the left-hand side is also $5.0 \, \text{N}$ and acts at the same perpendicular distance of $8.0 \, \text{cm}$ from the axle. Both forces produce a clockwise turning effect.
The moment about the axle produced by the force on the left-hand side is $\boxed{0.40 \, \text{Nm}}$.

a) (iii) Calculate the total moment on the axle.

Since both moments are in the same direction (clockwise), they add up.

The total moment on the axle is $\boxed{0.80 \, \text{Nm}}$.

b) (i) State the effect on the total moment about the axle of increasing the current in the coil.
Increasing the current in the coil will increase the force acting on the coil sides. Since moment is directly proportional to force ($M = Fd$), an increase in force will lead to an increase in the total moment.
The total moment will $\boxed{\text{increase}}$.

b) (ii) State the effect on the total moment about the axle of decreasing the number of turns of wire in the coil.
The force on a coil is directly proportional to the number of turns of wire. If the number of turns decreases, the force acting on the coil sides will decrease. Consequently, the total moment will decrease.
The total moment will $\boxed{\text{decrease}}$.

c) Name a device or a piece of apparatus which relies on the turning effect on a coil in a magnetic field.
A device that relies on the turning effect on a coil in a magnetic field is an $\boxed{\text{electric motor}}$.