Final answer:
The best variable to solve for via substitution is typically one that is already isolated; therefore, you would choose to solve either for option C (y in the 1st equation) or D (y in the 2nd equation), depending on which equation is simpler for substitution.
Step-by-step explanation:
To choose the best variable to solve for via substitution in a system of equations, we usually look for an equation where a variable is isolated or can be easily isolated. Although the specific system of equations is not provided in the details, the best variable to solve for would typically be one that appears with a coefficient of 1 or -1 to simplify the substitution process. If any of the provided equations already have y in the form y = mx + b (where m and b are constants), it would be most straightforward to substitute for y, because it is already isolated and we can directly plug it into the other equation. As such, if either the first or second equation has y isolated and looks like y = mx + b, solving for y from that equation would be the best choice. Therefore, without loss of generality, option C or option D would generally be the preferred choices, depending on which equation has the simpler form for substitution.