It is definitely C: 20%
Subtract the original value from the new value, then divide the result by the original value then multiply the result by 100 and it will give you the answer
[tex] \frac{4200 - 3500}{3500} \times 100[/tex]
[tex] \frac{700}{3500} \times 100[/tex]
[tex]0.2 \times 100 = 20\%[/tex]