To understand why the tearing efficiency of the joint is 75% when the ratio of the diameter of the rivet hole to the pitch of rivets is 0.25, let's break down the concept and calculations involved.

Tearing Efficiency:
Tearing efficiency is a measure of how effectively a joint can resist the tearing force. It is the ratio of the actual tearing force to the maximum possible tearing force. The tearing force is the force required to tear the plate along the line of the rivets.

Pitch and Diameter of Rivet Hole:
The pitch of rivets refers to the distance between the centers of two adjacent rivets. The diameter of the rivet hole is the diameter of the hole drilled in the plates to accommodate the rivet.

Calculation:
Given that the ratio of the diameter of the rivet hole to the pitch of rivets is 0.25, we can calculate the tearing efficiency using the following steps:

Step 1: Calculate the tearing force (T):
The tearing force (T) can be calculated using the formula:
T = (π/4) * d^2 * t * σt
where d is the diameter of the rivet hole, t is the thickness of the plate, and σt is the tensile strength of the plate material.

Step 2: Calculate the maximum possible tearing force (Tmax):
The maximum possible tearing force (Tmax) can be calculated using the formula:
Tmax = (π/4) * (d + 2p)^2 * t * σt
where p is the pitch of the rivets.

Step 3: Calculate the tearing efficiency (η):
The tearing efficiency (η) can be calculated using the formula:
η = (T / Tmax) * 100

Given that the ratio of the diameter of the rivet hole to the pitch of rivets is 0.25, it means that d = 0.25p.

Substituting this value in the formulas, we get:
T = (π/4) * (0.25p)^2 * t * σt
Tmax = (π/4) * (0.25p + 2p)^2 * t * σt

Simplifying these equations, we get:
T = (0.0625πp^2) * t * σt
Tmax = (3.765625πp^2) * t * σt

Finally, substituting the values of T and Tmax in the tearing efficiency formula, we get:
η = [(0.0625πp^2)/(3.765625πp^2)] * 100
η = 0.0166 * 100
η = 1.66%

Therefore, the tearing efficiency of the joint is 1.66%, which is approximately 2%. However, none of the given options match this result. Therefore, there may be an error in the given options, and the correct answer cannot be determined from the given options.