The correct answer is C. Athlete.
Step-by-step explanation
A base word is one unit of language that transforms in other words when prefixes or suffixes are added. In most cases, the pronunciation of the base word and its related words or similar, but sometimes the pronunciation can be different. This happens with the word athlete because this is pronounced as /ˈæθlit/ and the word athletic is pronounced /æθˈlɛtɪk/. According to the above, the correct answer is C. Athlete.