SAT Question dd4ab4c4: Equivalent Expressions answer and explanation with Desmos solution

Choice B is correct. The first and last terms of the polynomial are both squares such that $4a^2=(2a{)}^2$ and $25b^2=(5b{)}^2$. The second term is twice the product of the square root of the first and last terms: $20ab=2\times 2a\times 5b$. Therefore, the polynomial is the square of a binomial such that $4a^2+20ab+25b^2=(2a+5b{)}^2$, and $(2a+5b)$ is a factor.