Which affix is placed within the core word to create compound formations?

The interfix is the correct choice because it serves as a connecting element within compound words. Unlike prefixes and suffixes, which are added to the beginning or end of a base word respectively, an interfix appears in the middle of two components to link them together. This is particularly useful in languages that form compound words by combining two or more root words. The presence of the interfix helps to facilitate pronunciation and maintain the flow of the newly created word, ensuring that both original words are still discernible while forming a cohesive unit.