The correct option is
C (1, 5, 9); (3, 4, 8); (2, 6, 7)
If you closely observe the figures,
Figures
1, 5, and
9 ⇒ Consist of curved lines only
Figures
3, 4, and
8 ⇒ Consist of both straight and curved lines
Figures
2, 6, and
7 ⇒ Consist of straight lines only
Thus, the correct way to classify these images would be (1, 5, 9); (3, 4, 8); (2, 6, 7)