Answer:
[tex]\large\boxed{D.\ \dfrac{practice\cdot days}{distractions^2}}[/tex]
Step-by-step explanation:
[tex]S=\dfrac{P\cdot t}{D^2}\\\\P\to practice\\t\to days\\D\to distractions\\\\S=\dfrac{P\cdot t}{D^2}\ \bigg(\dfrac{practice\cdot days}{distractions^2}\bigg)[/tex]
