To solve this question, the user needs to understand what inspections are and what they are used for. Inspections are a type of static testing that involves a team of individuals examining code, documents, or other deliverables to identify defects or areas for improvement.

Now, let's go through each option and explain why it is right or wrong:

A. Variables not defined in the code: This option is correct. Inspections can help identify variables that have not been defined in the code, which can cause errors or unexpected behavior when the code is executed.

B. Spelling and grammar faults in the documents: This option is correct. Inspections can also help identify spelling and grammar errors in documents, which can affect the readability and clarity of the document.

C. Requirements that have been omitted from the design documents: This option is correct. Inspections can also help identify requirements that have been omitted from the design documents, which can lead to incomplete or incorrect designs.

D. How much of the code has been covered: This option is incorrect. Inspections are not typically used to determine code coverage. Code coverage is typically measured using tools such as test coverage analyzers or profiling tools.

Therefore, the answer is: D. How much of the code has been covered.