Final answer:
The correct use of 'reticent' in the given context means being 'Reserved' as it conveys a reluctance to share personal information.
Step-by-step explanation:
The sentence that best matches the context of the word reticent as it is used in the example would be "Reserved". This is evident from the usage of the word in sentences where individuals are not willing to share their personal thoughts or emotions freely. In the given instance, the moment the topic of dating is raised, the person becomes extremely reticent about his personal life, indicating a sudden switch to being reserved or unwilling to disclose personal details.