Angle a: This angle has a square symbol at its vertex, indicating it measures exactly 90^.

Here are the classifications for each angle:

• Angle a: This angle has a square symbol at its vertex, indicating it measures exactly $90^\circ$.

  • Type: Right angle

• Angle b: This angle is wider than $90^\circ$ but less than $180^\circ$.

  • Type: Obtuse angle

• Angle c: This angle is narrower than $90^\circ$.

  • Type: Acute angle

• Angle d: This angle has a square symbol at its vertex, indicating it measures exactly $90^\circ$.

  • Type: Right angle