The relationship between ∠a and ∠b in the figure is vertical. This is because they are formed by two intersecting lines that create perpendicular angles. Vertical angles are always equal to each other, so ∠a and ∠b would have the same measure.
Complementary angles: These angles add up to 90 degrees. From the image, we can see that ∠a and ∠b are not together 90 degrees, so they are not complementary.
Linear pair/supplementary angles: These angles add up to 180 degrees. Again, from the image, we can see that ∠a and ∠b are not together 180 degrees, so they are not supplementary.
Vertical angles: These are formed by two intersecting lines and are always equal to each other. In the image, we can see that ∠a and ∠b are formed by two intersecting lines, so they are vertical angles.
Therefore, the final answer is: vertical angles.