Gọi hóa trị R là a(a>0)
\(R_1^a\left(OH\right)_3^I\\ \Rightarrow a\cdot1=I\cdot3\Rightarrow a=3\Rightarrow R\left(III\right)\\ CTTQ:R_x^{III}O_y^{II}\\ \Rightarrow x\cdot III=y\cdot II\Rightarrow\dfrac{x}{y}=\dfrac{2}{3}\Rightarrow x=2;y=3\\ \Rightarrow R_2O_3\)
Chọn D