The correct answer is D. no article.
The word "office" is a common noun, which means it refers to a general class of things. In this case, the office is not a specific office, but rather any office. Therefore, we do not need to use an article.
If we were to use an article, it would imply that we are referring to a specific office. For example, if we said "I left it at the office," that would mean that we are referring to a specific office that the listener knows about.
However, in this case, we are not referring to a specific office, so we do not need to use an article.
