Explanation:

A lever is a simple machine that consists of a rigid rod or beam that can rotate around a fixed point called the fulcrum. The lever is used to exert a force to move or lift objects.

Components of a Lever:
A lever consists of the following components:
1. Fulcrum: The fulcrum is the fixed point around which the lever rotates. It acts as a pivot for the lever.
2. Rod: The rod is the rigid structure or beam that is used to apply force.
3. Load: The load is the object or weight that needs to be lifted or moved using the lever.

Working of a Lever:
When a force is applied to one end of the rod, it creates a moment or turning effect around the fulcrum. This moment causes the lever to rotate, and the load on the other end of the rod is lifted or moved.

Options:
Now let's analyze each option given in the question and see if it matches the components of a lever.

a) Fulcrum and a stone: This option mentions a fulcrum and a stone, but it does not mention a rod. A stone alone cannot form a lever, so this option is incorrect.

b) A rod and a fulcrum: This option mentions both a rod and a fulcrum, which are essential components of a lever. When a force is applied to the rod, it can rotate around the fulcrum, allowing the load to be lifted or moved. Therefore, this option is correct.

c) A stone and an iron rod: This option mentions a stone and an iron rod, but it does not mention a fulcrum. Without a fulcrum, the stone and iron rod cannot form a lever. Hence, this option is incorrect.

d) Can be all of the above: This option suggests that any combination of the mentioned components can form a lever. However, as we have seen, only option b (a rod and a fulcrum) is the correct combination for a lever. Therefore, this option is incorrect.

Conclusion:
The correct answer to the question is option b, which states that a lever consists of a rod and a fulcrum.