A. is the largest number.
3.2 * 10^5 = 320,000
4.2 * 10^3 = 4,200
1.1 * 10^5 = 110,000
A quicker way to solve it is to look at the exponents and see which one is bigger (in this case, the exponent 5 in A and C is bigger than the 3 in B) and then look at the decimal to see which is bigger (in this case, 3.2 is larger than 1.1). But yea, that's just my way to solve it.
I hoped this helped! :)