To get the speed s in therms of the variables d, i and t, que have to clear s from the expression we're given, as following:
[tex]\begin{gathered} d=i+st \\ \rightarrow d-i=st \\ \rightarrow\frac{d-i}{t}=s \end{gathered}[/tex]This way, the correct expression for s in therms of the variables d, i and t is:
[tex]s=\frac{d-i}{t}[/tex](Ans: Option A)