If a = 7, b = 5 and c = 3, then the value of $a^{2}+b^{2}+c^{2}-ab-bc-ca$ is:
[A]0
[B]8
[C]-12
[D]12

12
Given Expression,
$a^{2}+b^{2}+c^{2}-ab-bc-ca$
$=> \frac{1}{2}\left [ \left ( a-b \right )^{2}+\left ( b-c \right )^{2}+\left ( c-a \right )^{2} \right ]$
$=> \frac{1}{2}\left [ \left ( 7-5 \right )^{2}+\left ( 5-3 \right )^{2}+\left ( 3-7 \right )^{2} \right ]$
$=> \frac{1}{2}\left ( 4+4+16 \right )$
$=> \frac{1}{2}\times 24 = 12$
Hence option [D] is the right answer.