Question

(a) one pair of vertical angles (b) one pair of angles that form a linear - pair (c) one pair of angles that are congruent (1) vertical angles: ∠ and ∠ (2) linear pair: ∠ and ∠ (3) congruent angles: ∠2 and ∠5

Explanation:

Step1: Recall vertical - angles definition

Vertical angles are formed when two lines intersect. They are opposite each other and are congruent. In the given figure, $\angle1$ and $\angle3$ are vertical angles.

Step2: Recall linear - pair definition

A linear pair of angles is a pair of adjacent angles whose non - common sides are opposite rays. In the given figure, $\angle1$ and $\angle2$ form a linear pair.

Step3: Recall congruent - angles example

Congruent angles have the same measure. $\angle4$ and $\angle6$ are congruent as they are vertical angles.

Answer:

(a) $\angle1$ and $\angle3$
(b) $\angle1$ and $\angle2$
(c) $\angle4$ and $\angle6$