Find bounds on the real zeros of the polynomial function f(x) = x⁴ - 48x² - 49.
a) -8 ≤ x ≤ 8,
b) -7 ≤ x ≤ 7,
c) -6 ≤ x ≤ 6,
d) -5 ≤ x ≤ 5