Answer:
The correct option is 1.
Explanation:
The according to the angle sum property of triangle, the sum of interior angles of a triangle is 180°.
Step 1: According to the student in triangle ABC,

Which is not correct because the sum of interior angles of a triangle is 180°. So, he did a mistake in step 1.
In triangle ABC,
... (1) (sum of angles of a triangle)
Step 2:
.... (2) (supplementary angles)
Step 3: Equating (1) and (2), we get

Stpe 4:

Therefore the correct option is 1.