对啦！

The three numbers are: a, b=a+1, c=sqrt(ab)=sqrt(a(a+1)) [ a(a+1) is always non negative.]

D = a^2 + b^2 + c^2

= a^2 + (a+1)^2 + (sqrt(a(a+1)))^2

= 3a^2 + 3a + 1

When a =1, sqrt(D) = sqrt(7) is not a rational number, hence the answer is D.

I take the easy one and the difficult ones are left to 蒋大公子。