The greatest integer function, also known as the floor function, is defined as the greatest integer less than or equal to a given number.
For \( x = -1.5 \):
- The greatest integer less than or equal to \(-1.5\) is \(-2\).
Thus, when \( x = -1.5 \), \( y = -2 \).
So, the correct response is \( y = -2 \).