Correct answer is: a) 125
Formula use: \left(a+b\right)^3=a^3+b^3+3.a.b\left(a+b\right)

Explanation

Here, given expression is :
2.75\times2.75\times2.75+2.25\times2.25\times2.25+15\times2.75\times2.25
Let,
2.75=a
2.25=b
Now, find the value :
2.75\times2.75\times2.75+2.25\times2.25\times2.25+15\times2.75\times2.25
\rightarrow\left(2.75\right)^3+\left(2.25\right)^3+3.5.\left(2.75\times2.25\right)
\rightarrow\left(2.75\right)^3+\left(2.25\right)^3+3.\left(2.75+2.25\right).\left(2.75\times2.25\right)
\rightarrow a^3+b^3+3\left(a+b\right)a.b
\rightarrow\left(a+b\right)^3
Now, putting the a & b value, we get :
\left(2.75+2.25\right)^3
\rightarrow\left(5\right)^3
\therefore 125
Ans: 125.