You can clearly eliminate 0.79d as an answer, because that is less than the amount that Ron's class donated. The last answer is also incorrect, because it states that Danny's class donated 21 more cents than Ron's class. The problem says that Danny's class donated 21% more than Ron's. The only way that d + 0.21 would be correct is if Ron's class donated $1. The problem does not say that. Therefore, we are left with two possibilities. The first choice is the correct one. Why?
21% more than Ron's class' donation would be the amount that Ron's class donated + an additional 21%. If Ron's class donated d dollars, then an additional 21% would be 0.21 * d = 0.21d. Hence, Danny's class donated a total of:
d + 0.21d = d(1 + 0.21) = 1.21d
