For those who are facing difficulty understanding the question, I think the question can be better transcribed as follows:

"25 students take a test and Kelly gets 6.2 points higher than the average score. If a teacher gives all the 25 students another 10 points question and it is only Kelly who could solve that question, how much is Kelly’s score higher than the 25 students’ average score now?"

Posted from my mobile device

Now the question becomes relatively clear to understand--

Kelly received marks, m1= avg1+6.2

kelly is the only one who correctly answered the 10 marks question, now in total
she obtained

m2=m1+10= avg1+6.2+10

now the average of the entire class would be, avg1+10/25

therefore, she would receive, m2- (avg1-10/25)
= avg1+6.2+10- avg1-10/25
= 15.8 more than the class average (D is the answer)