Đáp án đúng: A
Giải chi tiết:\(\begin{array}{l}\overline {ab} \times 21 = \overline {9ab} \\\overline {ab} \times 21 = 900 + \overline {ab} \\\overline {ab} \times 21 - \overline {ab} = 900\\\,\,\,\,\,20 \times \overline {ab} \,\,\,\,\,\, = 900\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\overline {ab} \,\,\,\,\,\, = 900:20\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\overline {ab} \,\,\,\,\,\, = 45.\end{array}\)
Chọn A.