Try (a)
It can't be a-b < 1  because 3 > 1 yet 3-1 is 2 which is not less than 1

Try (b)
It can't be ab > b because 1 > 0 yet 1*0 is 0 which is not greater than 0

Try (d)
It can't be a + b > 3b because 3 > 2 yet 3+2 is 5 which is not greater 
than 3*2 which is 6

So it must be (c).  a + 1 > b for if a is larger than b, adding 1 to the
larger one, a, can only make it EVEN MORE LARGER than b.

(Is it correct to say "MORE LARGER"?  Must be! :)

Edwin