fundamental, basic

base, basis, foundation, ground, root / (construction, architecture) foundation

high-class, advanced

height, altitude / high class, high grade, high degree

solution, settlement / (music) Progression from dissonance to consonance; a chord to which such progression is made; resolution

(transitive) solve, resolve, settle (a problem, a trouble, a criminal case) / (intransitive) be solved, be settled / (intransitive) can be solved; be soluble

knowledge

needed, necessary

need, necessity

mathematics (field of study)