The reason given is:
[tex]\text{exterior }\angle=\text{ sum of remote interior }\angle^{\prime}s[/tex]
This means that an exterior angle is equal to the sum of the non adjacent interior angles.
In this case the non adjacent angles are 2 and 3, then the statement will be:
[tex]\angle1=\angle2+\angle3[/tex]
hence the answer is the first option.
