Q.Which of the following does not react with oxygen directly?
Zn, Ti, Pt, Fe

Dioxygen, $O_2$, is prepared in the laboratory by heating potassium chlorate with $MnO_2$ catalyst or by thermal decomposition of other oxysalts, and it combines directly with most elements to form oxides, though a few unreactive metals such as gold and platinum do not combine with oxygen directly under ordinary conditions. Combustion reactions such as $C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O$ and $4Al + 3O_2 \rightarrow 2Al_2O_3$ illustrate oxygen's strong oxidising power. Oxides are classified as acidic (non-metal oxides like $SO_2$ and $CO_2$), basic (metal oxides like $Na_2O$), amphoteric ($Al_2O_3$, $ZnO$) or neutral ($CO$, $NO$). Oxygen forms the oxide ion $O^{2-}$ far more readily than the singly-charged $O^{-}$ ion in stable solid compounds: converting $O^{-}$ further to $O^{2-}$ requires an extra input of energy (the second electron-gain step, …