To answer this question, you need to understand the concept of fault masking.
Option A) Error condition hiding another error condition - This option is correct because fault masking refers to a situation where one error condition hides or masks another error condition. In other words, the presence of one fault in a system may prevent the detection of another fault.
Option B) Creating a test case which does not reveal a fault - This option is incorrect because fault masking is not related to creating test cases. It is about the presence of one error condition hiding another error condition.
Option C) Masking a fault by a developer - This option is incorrect because fault masking is not done intentionally by a developer. It is a situation that can occur unintentionally in a system when one fault hides another fault.
Option D) Masking a fault by a tester - This option is incorrect because fault masking is not done intentionally by a tester. It is a situation that can occur unintentionally in a system when one fault hides another fault.
The correct answer is A) Error condition hiding another error condition. This option is correct because it accurately describes the concept of fault masking.