B. Writing an equation is the most useful
We are required to find the best method which can be used to find the value of a variable.
Since, we know,
While writing an equation, we assign the dependent variable in terms of the independent variable.
These equations can be easily solved using the methods such as substitution and elimination.
Thus, they give the exact value of the variables.
While options A, C and D, does not give the value of the variables over the domain.
So, the best option useful to find the value of the variable is to write an equation.
That is, option B is the most useful.