According to Quiz 2, the second and third matrices presented are valid rotation matrices. For this to be true, the traverse of the matrix should be the same as its inverse, which means that multiplying the matrix by its transverse should yield identity. That is absolutely not the case with the matrix in the second row.
thecarlos
commented
5 years ago 
According to Quiz 2, the second and third matrices presented are valid rotation matrices. For this to be true, the traverse of the matrix should be the same as its inverse, which means that multiplying the matrix by its transverse should yield identity. That is absolutely not the case with the matrix in the second row.
Lesson URL: https://classroom.udacity.com/nanodegrees/nd209/parts/c199593e-1e9a-4830-8e29-2c86f70f489e/modules/8855de3f-2897-46c3-a805-628b5ecf045b/lessons/87c52cd9-09ba-4414-bc30-24ae18277d24/concepts/613a7740-3575-4384-a2b2-c9f0b18924b4