Concept:The correct phrase used for officially entering a position of office is "sworn into office."
Explanation:The sentence refers to a chairman taking up his role formally.
"Sworn into office" is the standard expression for making a formal promise or oath.
The other options do not fit the fixed phrase "into office":
One is "installed" in a position, not into office.
One is "admitted" to a group or place, not to office.
One is "inaugurated" as an event, but the word that completes the given phrase is "sworn."
Answer:A. sworn