Option: d is the correct answer.
d. Square of a Binomial
Binomial--
Binomial is a expression consisting of two terms.
( since bi basically means two)
Now here we have a identity as:
[tex](3+6)^2=81[/tex]
i.e. there are two terms 3 and 6 which are separated by a addition sign and hence we square this binomial to get the desired value.
Since 6+3=9
and [tex](6+3)^2=9^2=81[/tex]
Hence, the answer is:
Square of a binomial.