What can be said regarding a straight line if its slope is negative?

1. Inclination, θ is an acute angle.

2. Inclination, θ is an obtuse angle.

3. Either the line is x-axis or it is parallel to the x-axis.

4. Either the line is y-axis or it is parallel to the y-axis.

If the slope is negative, then tan θ must be negative.

tan θ < 0

This happens only when the angle θ made with the positive x-axis is between 90° and 180°.

And angles between 90° and 180° are called obtuse angles.

Hence, option 2 is the correct option.