A polynomial is in standard form when its terms are arranged in descending order of their degrees (the highest power of the variable comes first).
Let's analyze each option:
A.
Terms are arranged as (6) (degree 0), () (degree 2), and (3x) (degree 1).
Not in standard form (highest degree term is second).
B.
Terms are arranged as () (degree 2), (3x) (degree 1), and (6) (degree 0).
Standard form (terms are in descending order).
C.