maths explain please!!

G.C.D = greatest common divisor = the greatest common factor (G.C.F) = highest common factor (H.C.F)

The highest common factor (HCF) of two whole numbers is the largest whole number which is a factor of both.

Example:
Consider the numbers 12 and 15:
The factors of 12 are : 1, 2, 3, 4, 6, 12.
The factors of 15 are : 1, 3, 5, 15.
1 and 3 are the only common factors (numbers which are factors of both 12 and 15).
Therefore, the highest common factor of 12 and 15 is 3.

So,
The G.C.D. of 20ab^2,40a^2bc and 80a^3c^2 is
B.20a

the G.C.D. of (x^2-y^2) and (x-y)(x+y)^2 is
C.(x-y)(x+y)

(x^2-y^2)= (x+y) (x-y)

Should you have any further questions, pls feel free to ask me!! :)))