Explanation:

Introduction:
In an external involute spur gear pair, tooth interference occurs when the teeth of the gear and pinion come into contact before the intended point of engagement. This interference can lead to noise, vibration, and premature wear of the gear teeth. Therefore, it is desirable to reduce tooth interference in gear pairs.

Factors affecting tooth interference:
Several factors can influence tooth interference in a gear pair, including the center distance between the gears, the module, the pressure angle, and the number of gear teeth.

Decreasing center distance:
When the center distance between the gears is decreased, the distance between the gear teeth also decreases. This reduction in distance may increase the likelihood of tooth interference because there is less space for the teeth to mesh properly. Therefore, decreasing the center distance would not be effective in reducing tooth interference.

Decreasing module:
The module refers to the size of the gear teeth. When the module is decreased, the gear teeth become smaller, and the distance between adjacent teeth decreases. This reduction in tooth size can help reduce tooth interference because there is more space for the teeth to mesh properly. Therefore, decreasing the module can be effective in reducing tooth interference.

Decreasing pressure angle:
The pressure angle determines the angle at which the gear teeth exert force on each other. When the pressure angle is decreased, the force exerted by the teeth is reduced. This decrease in force can help reduce tooth interference because there is less chance of the teeth coming into contact prematurely. Therefore, decreasing the pressure angle can be effective in reducing tooth interference.

Increasing number of gear teeth:
When the number of gear teeth is increased, the teeth become smaller, and the distance between adjacent teeth decreases. This reduction in tooth size can help reduce tooth interference because there is more space for the teeth to mesh properly. Therefore, increasing the number of gear teeth can be effective in reducing tooth interference.

Conclusion:
Out of the given options, the most effective way to reduce tooth interference in an external involute spur gear pair is by increasing the number of gear teeth. This allows for smaller teeth and more space for proper meshing, reducing the likelihood of tooth interference.