Using the binomial theorem expression, what is the expansion of (x + 3y)⁵?
a) (x⁵ + 15x⁴y + 90x³y² + 270x²y³ + 405xy⁴ + 243y⁵)
b) (y⁵ + 15y⁴x + 90y³x² + 270y²x³ + 405yx⁴ + 243x⁵)
c) (x⁵ + x⁴y + x³y² + x²y³ + xy⁴ + y⁵)
d) (x⁵ - 15x⁴y + 90x³y² - 270x²y³ + 405xy⁴ - 243y⁵)
