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Mastering basic mathematics is an important goal for younger students.
A. Mastering
B. Important
C. Younger
D. Students
D: Students is a plural noun. NOTE: Mastering (A) is a gerund, i.e. a verb form functioning as a noun. But since (D) is already a noun, it is the better choice. Important (B) is an adjective modifying the noun goal. Younger (C) is an adjective modifying the noun students.