Final answer:
The first step in solving a system of linear equations using the substitution method is to solve one of the equations for one of its variables before substituting it into other equations.
Step-by-step explanation:
When using the substitution method for solving a system of linear equations in three variables, the first step should be to solve one equation for one of its variables (Option C). This involves selecting one of the equations and isolating one variable. Once a single variable is expressed in terms of the other variables, this expression can be substituted into the other equations. This substitution simplifies the system by reducing the number of variables in those equations, which can then be solved using further substitutions or other methods such as elimination or graphing.