3(a2 + b2 + c2) = (a + b + c)2 then the relation between a, b and c is ?

Answer: Option D

Always do these types of question with the help of,
$$\begin{array}{rl}& \text{Put }a=b=c=1\\ & \text{3}\left(a^2+b^2+c^2\right)={\left(a+b+c\right)}^2\\ & 3=3\text{ Satisfied}\\ & \text{So, this is answer}\to a=b=c\end{array}$$