SAT Question e1248a5c: Systems of Linear Equations answer and explanation with Desmos solution

Choice D is correct. A system of two linear equations has infinitely many solutions if one equation is equivalent to the other. This means that when the two equations are written in the same form, each coefficient or constant in one equation is equal to the corresponding coefficient or constant in the other equation multiplied by the same number. The equations in the given system of equations are written in the same form, with $x$ and $y$ on the left-hand side and a constant on the right-hand side of the equation. The coefficient of $y$ in the second equation is equal to the coefficient of $y$ in the first equation multiplied by $3$. Therefore, $a$, the coefficient of $x$ in the second equation, must be equal to $3$ times the coefficient of $x$ in the first equation: $a=\frac{1}{2}\times 3$, or $a=\frac{3}{2}$.