Concept:The underlined word is
vigilant, which means keeping careful watch for possible danger or difficulties.
Explanation:A vigilant person stays alert and observant at all times.
Among the options,
watchful directly matches this meaning of paying close attention.
Intelligent means having sharp mental ability, not alertness.
Rigid means stiff or inflexible, which is unrelated.
Concerned suggests worry or interest, not constant attention.
Thus, the closest synonym for
vigilant is
watchful.
Answer:B. watchful