If \[{{3}^{m}}=\text{ }{{\left( 81 \right)}^{n}}=\text{ }{{\left( 729 \right)}^{r}}=\text{ }243\] then find the value of \[{{\left( \frac{3m}{4}+n \right)}^{3}}-{{(4n+6r)}^{3}}\]

• A.  1125                  
• B.      875
• C.  -875            
• D.     -1125
• E.  None of these