Correct Option: A. vital Explanation of the Correct Answer: The word "indispensable" means something that is absolutely necessary or essential. When we say that Bisi has become an "indispensable" member of staff, we are emphasizing that her presence and contributions are crucial to the functioning of the team or organization. The option "A. vital" is the best synonym for "indispensable" because "vital" also conveys the idea of something being essential or necessary for survival or success. In this context, if Bisi is vital to the team, it means that without her, the team would struggle to perform effectively. Why the Other Options are Incorrect or Weaker:

• B. principal: This word typically refers to someone who has the highest authority in an organization, such as a school principal, or it can mean the most important or primary. While "principal" can imply importance, it does not capture the essence of being absolutely necessary or essential in the same way "indispensable" does. Therefore, it is not a suitable synonym.
• C. dedicated: This term describes someone who is committed or devoted to a task or purpose. While a dedicated member of staff can be important, dedication does not inherently mean that the person is essential or cannot be replaced. Thus, "dedicated" does not convey the same level of necessity as "indispensable."
• D. effective: This word refers to the ability to produce a desired result or outcome. While being effective is certainly a positive trait for a staff member, it does not imply that the person is essential or cannot be done without. An effective employee can still be replaced, which is contrary to the meaning of "indispensable."

Summary of Key Points:

• "Indispensable" means absolutely necessary or essential.
• The correct synonym is "vital," which also conveys the idea of being crucial.
• "Principal" refers to authority or importance but lacks the necessity aspect.
• "Dedicated" indicates commitment but does not imply essentiality.
• "Effective" relates to producing results but does not mean someone is irreplaceable.

By understanding these nuances, you can better grasp the meanings of words and their synonyms, which is essential for vocabulary and comprehension in English.