Explanation

Step 1: Recall the definition of probability

Probability is a measure between 0 and 1, inclusive.

Step 2: Evaluate the options

15% = 0.15 (between 0 and 1)

2/3 ≈ 0.67 (between 0 and 1)

-0.5 (less than 0)

0.01 (between 0 and 1)

Step 3: Determine which option is outside the valid range

-0.5 is less than 0, which is not valid for a probability.