A common factor is just a value that both numbers can be divided by to give you an integer. (an integer is a number without a decimal point btw or anything that is not a fraction)
Answer A and B are too small so there's no way they can be divided by 10.
D and E are too big, so they can be divided by a number bigger than 10. (the question did ask for the biggest dividing number to be 10)
