Đáp án đúng: B
Giải chi tiết:\(\begin{array}{l}\,\,\,{4^{x + 3}} + {4^x} = 1040\\\,{4^x}{.4^3} + {4^x} = 1040\\{4^x}.\left( {{4^3} + 1} \right) = 1040\\\,\,\,\,\,\,\,\,\,\,\,{4^x}.65 = 1040\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{4^x} = 16\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{4^x} = {4^2}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x = 2\end{array}\)
Vậy \(x = 2\).
Chọn B.