The word "reply to" is commonly used in communication, but there are several synonyms that can be used to convey the same meaning. "Respond to" is a suitable alternative, and it can be used in both formal and informal contexts. "Reply back" is also frequently used, though it is slightly more informal. "Answer" and "acknowledge" are synonyms that indicate the recipient has received the message, but may not necessarily be responding to it. "Retort" and "rebut" indicate a more defensive or argumentative response. Ultimately, the synonym chosen depends on the tone and intention of the response in the given context.