Đáp án đúng: C
Giải chi tiết:\(\begin{array}{l}\,\,\,\,\,\,\,\,\,P = \left( {a - 1} \right)\left( {b - 2} \right) - \left( {ab + 2} \right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \left[ {a\left( {b - 2} \right) - 1.\left( {b - 2} \right)} \right] - \left( {ab + 2} \right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \left[ {\left( {ab - 2a} \right) - \left( {1.b - 1.2} \right)} \right] - \left( {ab + 2} \right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \left( {ab - 2a - b + 2} \right) - \left( {ab + 2} \right)\\\,\,\,\,\,\,\,\,\,\,\,\,\,\, = ab - 2a - b + 2 - ab - 2\\\,\,\,\,\,\,\,\,\,\,\,\,\,\, = - 2a - b\\ \Rightarrow P = - 2a - b\end{array}\)
Chọn C.