To find out the strength of a rivet in bearing, we need to multiply the permissible bearing stress by the expression (p - d)t, where p is the pitch, d is the diameter of the rivet hole, and t is the thickness of the plate being joined.

Explanation:
The bearing strength of a rivet refers to its ability to withstand the pressure or load applied to it when it is in contact with the material it is joining. It is important to calculate this strength accurately to ensure that the rivet will not fail under the given load.

The formula for the bearing strength of a rivet is given by the expression (p - d)t, where p is the pitch, d is the diameter of the rivet hole, and t is the thickness of the plate being joined.

Let's break down the components of this expression:

1. Pitch (p): The pitch of a rivet refers to the distance between the centers of two adjacent rivets. It represents the spacing of the rivets along the joint. It is important to have an appropriate pitch to distribute the load evenly across the joint.

2. Diameter of the rivet hole (d): The diameter of the rivet hole is the size of the hole drilled in the material to accommodate the rivet. It should be slightly larger than the diameter of the rivet to allow for proper insertion and expansion of the rivet.

3. Thickness of the plate (t): The thickness of the plate being joined is an important factor in determining the strength of the rivet. Thicker plates generally require larger and stronger rivets.

The expression (p - d)t represents the effective area of the material that is in contact with the rivet. It is derived by subtracting the hole diameter from the pitch and multiplying it by the plate thickness.

By multiplying the permissible bearing stress with this expression, we can determine the maximum load that the rivet can withstand before it fails in bearing.

Therefore, the correct answer is option 'A' - (p - d)t.