API testing involves testing the application programming interfaces directly and is often a critical part of integration testing. Here are the clarifications for each option:

A . API testing cannot be applied to distributed systems: This is incorrect. API testing is commonly used in distributed systems to ensure that different components interact correctly.

B . Combinatorial testing cannot be applied to API testing: This is incorrect. Combinatorial testing techniques can be very effective in API testing to cover various parameter combinations.

C . API testing may require the use of specialized tools: This is true. API testing often involves tools like Postman, SoapUI, and others to facilitate sending requests and validating responses, which may not be feasible manually.

D . APIs are loosely coupled, so timing glitches will not happen: This is incorrect. Even though APIs are designed to be loosely coupled, timing issues can still occur, especially in asynchronous operations and in systems where timing is critical.

Therefore, the correct answer is C. API testing may require the use of specialized tools.