Which statement best defines the least common denominator (LCD) for rational expressions?

Factor the numerical denominators $84$ and $36$ completely into prime factors, then find the LCD of $\frac{1}{84}$ and $\frac{1}{36}$.

Find the LCD for the denominators: $8x$, $12x^2$, and $18x^3$.

Add the rational expressions: $\frac{2}{x+3}+\frac{x}{x^2+6x+9}$

Factor the quadratic denominator $x^2+7x+12$ into irreducible factors over the integers.