The common factors of 10a, 20ab and 30bc are:

10

A common factor is a whole number which is a factor of two or more numbers. The highest common factor (HCF) is the greatest factor that will divide into two or more numbers. The lowest common multiple (LCM) is the smallest multiple that is common to two or more numbers.

Out of the given options, 10 is the common factor among the 3 numbers.