To solve this question, the user needs to have knowledge about software testing and the types of testing performed during the software development life cycle.

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

A. This option is incorrect because although it is true that a multi-user system may experience field failures, it is not an acceptable risk. All software systems should be designed and tested to handle multiple users accessing the system simultaneously.

B. This option is incorrect because insufficient functional testing may cause defects or issues in the system's functionality. However, a field failure occurring due to multiple users accessing the system is not a functional issue but a non-functional one.

C. This option is correct. A field failure occurring when multiple users access a system indicates that an important non-functional requirement was not specified and tested. Non-functional requirements include performance, usability, security, and reliability. A multi-user system should be designed and tested to handle the expected number of users accessing the system simultaneously.

D. This option is incorrect because it is possible to test against such events prior to release. Load testing and stress testing are types of non-functional testing that can be performed to test a system's performance when multiple users access the system simultaneously.

Therefore, the correct answer is:

The Answer is: C