What did I do wrong??

Choice A would be true if it was for 8 years, and not 8 months. The value of t is in years. We would need to convert 8 months to years like so

12 months = 1 year

1 month = 1/12 of a year

8 months = 8/12 of a year

8 months = 2/3 of a year after dividing both parts by 4

So we'll go for choice B as the final answer