In order to balance a chemical equations, the amount of atoms on the left side must be equ to the amount of atoms on the right side. There are 3 O atoms on the left side and 2 on the right. This means you should write 2 in front of KClO3 and 3 in front of O2.
2KClO3 - > KCl + 3O2
Now there are 6 O atoms on both sides. But now there are 2 K and CL atoms on the left and only one of each on the right. To balance that, you should write 2 in front of KCl, too. At the end, the equation should look like this:
2KClO3 - > 2KCl + 3O2
The answer is b) 2
