To answer this question, we need to understand the different types of faults that can lead to software failure.
Option A) 2 is a valid reason; 1,3,4&5 are not - This option is incorrect. While software faults (Option 2) can certainly lead to failure, other types of faults can also be responsible.
Option B) 1,2,3,4 are valid reasons; 5 is not - This option is incorrect. Documentation faults (Option 5) can also contribute to software failure.
Option C) 1,2,3 are valid reasons; 4&5 are not - This option is incorrect. Environment faults (Option 4) can also cause software failure.
Option D) All of them are valid - This option is correct. All the mentioned faults (1. Testing fault, 2. Software fault, 3. Design fault, 4. Environment fault, 5. Documentation fault) can potentially lead to software failure. Each of these faults can have a different impact on the software's functionality and performance.
Therefore, the correct answer is option D) All of them are valid.