The most suitable word to fill the gap in the passage is a) secretaries.
The sentence would read: "This may happen if his own country cannot afford to spend too large a part of its financial resources on the salaries of its secretaries in foreign countries."
While "delegates," "representatives," and "appointees" could also work in some contexts, "secretaries" specifically refers to officials who might be responsible for administrative tasks in foreign countries, making it the most appropriate choice here. "Refugees" does not fit the context of salaries in foreign countries.