Resposta
If $D > 0$, there are 2 distinct real solutions; if $D = 0$, there is exactly 1 real solution (tangent to x-axis); if $D < 0$, there are no real solutions.
Explicação
On the Digital SAT, questions asking for what value of $k$ the equation has exactly one solution frequently require setting $b^2 - 4ac = 0$ and solving for $k$.