A wall is ________ to an adjacent wall when they are perpendicular to each other.

• A. at a $\mathrm{straight} \mathrm{angle}$
• B. at an $\mathrm{obtuse} \mathrm{angle}$
• C. at a $\mathrm{right} \mathrm{angle}$
• D. at an $\mathrm{acute} \mathrm{angle}$

Answer: (C)

Verified step by step guidance

1. Understand the meaning of the term 'perpendicular' in geometry and trigonometry: two lines or walls are perpendicular if they meet at a right angle.
2. Recall that a right angle measures exactly 90 degrees, which is the defining characteristic of perpendicularity.
3. Recognize that an acute angle is less than 90 degrees, an obtuse angle is greater than 90 degrees but less than 180 degrees, and a straight angle is exactly 180 degrees.
4. Since perpendicular walls meet at a 90-degree angle, they are said to be at a right angle to each other.
5. Therefore, the correct completion of the sentence is: 'A wall is at a right angle to an adjacent wall when they are perpendicular to each other.'