جای خالی را پر کنید

To fill in the blank with a suitable word, we need to consider the context of the sentence. The sentence is connecting two ideas: 1. "**My mother is...**" 2. "**she is in the office**" A common word that would logically fit in this context is "because," which shows a cause-and-effect relationship. The sentence would read: "My mother is **at work because** she is in the office." This explains that the reason she is in the office is that she is at work.