Final answer:
The main issue with developers testing their own code is a lack of objectivity, which might lead to overlooking flaws due to familiarity with the code or an inclination to avoid discovering errors.
Step-by-step explanation:
The biggest problem with a developer testing his own code is C. Developers are not objective about their own code. This lack of objectivity can lead to a bias called 'blindness to one's own errors,' where the developer might overlook flaws because they are too familiar with the logic and flow of the code they've written. Additionally, there might be an unconscious inclination to not press too hard in testing to avoid discovering errors that could imply more work or weaknesses in their own skill set.