The correct option is C Olecranon process of radius ulna
The semilunar notch or the trochlear notch as it is commonly referred to is a greater sigmoid cavity formed by a large depression in the upper extremity
of the ulna. This fits the trochlea of the humerus and becomes a part of the elbow joint. This is formed by the olecranon and the coronoid process. Thus, the correct answer is option C.