If [z] denotes the greatest integer less than or equal to z and [z] = -1 which of the following statements must be true?

[BTGModeratorVI]

If [z] denotes the greatest integer less than or equal to z and [z] = -1 which of the following statements must be true?

A. z = -1
B. -2 <= z < -1
C. -2 < z <= -1
D. -1 <= z < 0
E. -1 < z <= 0

Answer: D
Source: GMAT prep

[Brent@GMATPrepNow]

If [z] denotes the greatest integer less than or equal to z and [z] = -1 which of the following statements must be true?

A. z = -1
B. -2 <= z < -1
C. -2 < z <= -1
D. -1 <= z < 0
E. -1 < z <= 0

Answer: D
Source: GMAT prep

Let's make sure we understand what this [] symbol means.
Here are a few examples:
[3.1] = 3
[5.8] = 5
[0.7] = 0
[-0.9] = -1
[-4.6] = -5

So, if [z] = -1, then -1 ≤ z < 0

Answer: D

Cheers,
Brent