The correct option is B watchful
“Vigilant” means keeping careful watch for possible danger or difficulties.
Hence option B is the correct answer because “to be watchful” means to carefully watch or keep an eye, hence it is correct.
Option A is incorrect because ‘smart’ means being intelligent and sharp, and is not related to being alert and watchful.
Option C is incorrect because ‘ambitious’ means to have high aims, which is not related here.
Option D is incorrect because “intelligent” means being sharp and smart, which is incorrect here as it is not related to being watchful.