Which of these is considered a defect that requires removal by grinding?

The choice identifying underfill as a defect that requires removal by grinding is valid, as underfill occurs when the weld metal does not completely fill the groove or joint, leading to a reduction in strength and potential performance issues. Addressing underfill by grinding is necessary to ensure that the weld meets standards for strength and integrity.

However, it's important to recognize that both holes in the weld and surface porosity also warrant attention and may require removal by grinding depending on their severity. Holes can compromise the structural integrity of the weld, while porosity can indicate trapped gas bubbles during the welding process, leading to weakness in the weld.

In the context of API 1104, a comprehensive understanding of various defects and their remediation methods is crucial. In many scenarios, correcting these defects, including surface porosity and holes in the weld, can indeed necessitate grinding. Hence, a broader view signals that all the defects listed should be addressed to ensure a sound weld without compromising safety and performance standards.