Đáp án đúng: C
Giải chi tiết:\(\begin{array}{l}\,\,{\left( {3x + 1} \right)^4} = 16.625\\\,\,{\left( {3x + 1} \right)^4} = {2^4}{.5^4}\\\,\,{\left( {3x + 1} \right)^4} = {10^4}\\\,\,\,\,\,\,\,\,3x + 1 = 10\\\,\,\,\,\,\,\,\,3x\,\,\,\,\,\,\,\, = 9\\\,\,\,\,\,\,\,\,\,\,x\,\,\,\,\,\,\,\, = 3\end{array}\)
Vậy \(x = 3.\)
Chọn C.