b. Angiosperm
Explanation:
Acorn is the fruit of oak tree. It is a nut and bears only one seed.
Since the scientist finds that the fruit produced on the tree is a nut then it is obviously an angiosperm because:
The only groups of seeded plant are Gymnosperms and Angiosperms.
Gymnosperms produce cones and have naked seed.
The seed produced within a fruit is a characteristic of angiosperm.
