The correct answer is: A. Effective throat thickness.
The effective throat thickness is the minimum dimension of a weld that determines its strength. It is measured perpendicular to the weld face and is equal to the minimum of the following:
- The root penetration
- The leg length
- The weld size
The root penetration is the depth of the weld into the base metal. The leg length is the distance from the weld face to the toe of the weld. The weld size is the width of the weld at its widest point.
The effective throat thickness is important because it determines the strength of the weld. A weld with a larger effective throat thickness will be stronger than a weld with a smaller effective throat thickness.
The other options are incorrect because they do not determine the strength of the weld. Plate thickness is the thickness of the base metal. Size of weld is the width of the weld at its widest point. Penetration thickness is the depth of the weld into the base metal.