The best option to model the equation \( 0.08x = 0.06x + 140 \) is:
A certain sum yields $140 more in interest when the interest rate is 8% than when it is 6%.
This response accurately reflects the relationship represented by the equation, where \( x \) is the principal amount, \( 0.08x \) is the interest earned at an 8% rate, and \( 0.06x + 140 \) accounts for the interest earned at a 6% rate plus an additional $140.