According to the syllabus, a defect report is a document that records the occurrence, nature, and status of a defect. A defect report typically includes several fields that provide relevant information about the defect, such as its title, description, priority, status, environment, steps to reproduce, expected result, actual result, etc. The answer A is correct because it suggests adding severity as an additional defect report field. Severity is a measure of the impact of a defect on the system or its users. Severity can help prioritize defects and decide whether they need to be fixed or not. The other answers are incorrect because they suggest adding fields that are already present in the defect report. Test script id is not necessary for a defect report, as it can be inferred from the steps to reproduce. Actual results and expected results are already included in the defect report under what occurred and what should have occurred.
References: Certified Tester Foundation Level Syllabus, Section 5.2.1, page 58-59.