Simple:
* She spent the afternoon replying to emails.
* He was replying to a comment on social media.
* The student was replying to the teacher's question.
More complex:
* Despite being exhausted, he continued replying to the barrage of questions.
* She was busy replying to her clients' concerns, hoping to resolve their issues quickly.
* He enjoyed replying to fan mail, feeling a connection to his supporters.
With different tenses:
* He's been replying to all his messages since he woke up.
* She will be replying to the email later this evening.
* They had been replying to each other for hours before finally deciding to meet.
Remember, "replying" is the present participle of the verb "reply," which means to answer or respond to something.