If you were to solve the following system by substitution, what would be the best
variable to solve for and from what equation?
2x+8v=12
3r- 8v=11

To solve the two equations simultaneously using the substitution method we need to rearrange one of the equation to make either  or  the subject. 

We can try in turn rearranging both equations and see which unknown term would have been easier to solve first

Equation 

Making  the subject
, dividing each term by 2
⇒ (Option 1)

Making  the subject
, multiply each term by 8 gives
⇒ (Option 2)

Equation 
Making  the subject
, divide each term by 3
 ⇒ (Option 3)

Making  the subject
, divide each term by 8
 ⇒ (Option 4)

From all the possibilities of rearranged term, the most efficient option would have been the first option, from equation  with  as the subject, 

I’d say letter a..