Chọn câu sai.

Ta có: $\left( { - 15{x^2}{y^6}} \right):\left( { - 5x{y^2}} \right)$$= \left( { - 15} \right):\left( { - 5} \right)\left( {{x^2}:x} \right)\left( {{y^6}:{y^2}} \right) = 3x{y^4}$  nên A đúng.

+ $\left( {8{x^5} - 4{x^3}} \right):\left( { - 2{x^3}} \right)$$= \left[ {8{x^5}:\left( { - 2{x^3}} \right)} \right] - \left[ {4{x^3}:\left( { - 2{x^3}} \right)} \right]$ $=  - 4{x^2} + 2$ nên B đúng.

+ $\left( {3{x^2}{y^2} - 4x{y^2} + 2x{y^4}} \right):\left( { - \dfrac{1}{2}x{y^2}} \right)$$= \left[ {3{x^2}{y^2}:\left( { - \dfrac{1}{2}x{y^2}} \right)} \right] - \left[ {4x{y^2}:\left( { - \dfrac{1}{2}x{y^2}} \right)} \right] + \left[ {2x{y^4}:\left( { - \dfrac{1}{2}x{y^2}} \right)} \right]$

$=  - 6x + 8 - 4{y^2}$ nên C sai.

+ ${\left( {3x - y} \right)^{19}}:{\left( {3x - y} \right)^{10}} = {\left( {3x - y} \right)^{19 - 10}} = {\left( {3x - y} \right)^9}$ nên D đúng.