What is the value for SIGN(NVL(-16,0))?

Here Let see how we calculate these values.

Consider we have the following : SIGN(ABS(NVL(-75,0)))

Just like mathematics we should take it from the inner brackets.

So first step will be,
NVL(-75,0) = -75

Now, next bracket

ABS(-75) = 75 //ABS always take positive

Finally SIGN(75) = 1
If SIGN(>1) = 1, SIGN(<1) = -1, SIGN(0) = 0.

So in our question we didn't use the ABS function, So it remains Negative and SIGN Function of Negative will be -1.