Final answer:
The degree of the polynomial function f(x) = 3x⁴ + x³ - 21x² is 4, making Option A the correct answer. The highest degree term, 3x⁴, determines the degree of the polynomial.
Step-by-step explanation:
The degree of a polynomial function is the highest power of the variable that appears in the polynomial. Given the function f(x) = 3x⁴ x³ - 21x², there appears to be a typographical error. However, if we assume that the function should be f(x) = 3x⁴ + x³ - 21x², then the term with the highest degree is 3x⁴, which has a degree of 4. Therefore, the correct answer is Option A: 4. This follows from the general principle that the degree of the polynomial is the largest integer powers of x present when the polynomial is expressed in its standard form. Note that all coefficients and constants in the polynomial must be considered when determining the degree.