Hướng dẫn:

                                      $N{{a}_{2}}O+{{H}_{2}}O\to 2NaOH$

                                      0,25       $\to $          0,5

                                 ${{n}_{_{N{{a}_{2}}O}}}=\frac{{{m}_{N{{a}_{2}}O}}}{{{M}_{N{{a}_{2}}O}}}=\frac{15,5}{62}=0,25$(mol)

                                  ${{C}_{{{M}_{NaOH}}}}=\frac{{{n}_{NaOH}}}{{{V}_{NaOH}}}=\frac{0,5}{0,25}=2$  (M)

                $\Rightarrow $ Chọn đáp án C.