An envelope that has the correct starting clue include the following: B. Envelope 2.
In Mathematics and Geometry, a rational number is a type of number which comprises fractions, integers, terminating or repeating decimals such as the 0, 2/3, 0.12345678901234567890, etc.
Generally speaking, a rational number is a type of number that is represented as p/q.
where:
p and q are integers
q ≠ 0.
Based on the information provided about the clue on each envelopes, we have the following;
Envelope 1 =

Envelope 1 = -16/0
Envelope 1 = undefined (Not a rational number).
Envelope 2 =

Envelope 2 = 0/-2
Envelope 2 = 0 (A rational number).
Envelope 3 =

Envelope 3 = undefined (Not a rational number).
Envelope 4 =

Envelope 4 = undefined (Not a rational number).
Fabrizio creates a scavenger hunt for his friend Milo. He hands Milo four envelopes, each with some writing on the outside, as shown below.
Envelope 1:

Envelope 2:

Envelope 3:

Envelope 4:

He tells Milo that the correct clue to start the hunt is inside the envelope with a rational number written on it. Which envelope has the correct starting clue?
A. Envelope 1
B. Envelope 2
C. Envelope 3
D. Envelope 4