PUZZLE 135 Trouble Sum

The professor has written a sum on the board, but it doesn't make any sense.
It can apparently be fixed by adding a curvy line in one location, but which curvy line is the right one?
Bear in mind that there is no need to rotate the line in question; you use it just how it is already displayed.

Hint 1

Try working on the section of the sum where the figures look wrong.
Which part looks strange?

Hint 2

You need to place the curvy line on the right-hand side of the sum, in other words, where the answer is.
What do you need to add to the answer to make the sum correct?

Hint 3

The two circles on the right-hand side of the sum are parts of numbers.

Super Hint

Leave the left-hand side of the sum as it is, and change the right-hand side to reflect the answer.
Which line do you need to use to form the correct answer?
When you are selecting the line, look out for lines that are of a similar shape.

Answer for 'Trouble Sum'

The answer is 'D'.