If a and b are odd integers, then a – b is an even integer.

a and b are not odd integers
a and b are odd integers
a – b is an even integer
a – b is not an even integer

So the answer choices are
1. a-b=even
2. a and b are not odd
3. a and b are odd
4. a-b=even
5. a-b=not even

even=not odd
not even=odd so choice 5 is really a-b=odd
basically choice 4 and 1 are the same so we cross one out
so
the problem said that a and b are odd so therefor choice2 is wrong and choice 3 is correct

then both are odd
odd-odd=even because
an even number is represented as 2n where n is an integer
an odd number can be represented as 2n+1 so assume you have 2 odd numbers 2 away from each other so odd and (odd+2)
odd+2-odd=2n+1+2-(2n+1)=2n+1+2-2n-1=2n-2n+1-1+2=2
you are left with odd
using integers
7 and 11
11-7=4
even

so odd-odd=even, it depends on weather you consider 0 odd or even

so the asnwers are:
a and b are odd
a-b is not an even integer