In many universities across the world, C programming is taught to novice students in the first programming course. To assist their self-studies, we have developed C programming learning assistant system (CPLAS) by extending works of JPLAS for Java. The recursive function is one of the most important but very hard concepts in C programming. It can make the code shorter by calling the function itself to solve the smaller problem. The understanding is essential in improving programming abilities of students. In this paper, we propose the phrase fill-in-blank problem (PFP) as a new type exercise problem in CPLAS for learning the recursive function. In a recursive function, the condition to terminate the recursive call, its returning value, and the argument to call the function are the key factors. Thus, in a PFP instance, their corresponding phrases (sets of elements) are blanked from a given source code, and are requested to be filled in by students. The correctness of answers is marked through string matching with the original ones. As preliminary evaluations, we generated 15 PFP instances and assigned them to 8 students in Okayama University. The results found the weakness of them.