(a) $\mathrm{CH}_3-\mathrm{CH}_2-\overset{\mathrm{OH}}{\overset{\parallel}{\mathrm{C}}}-\mathrm{CH}_2-\mathrm{CH}_3$ and $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_2-\overset{\mathrm{OH}}{\overset{\parallel}{\mathrm{C}}}-\mathrm{CH}_3$ are structural isomers. Structural isomerism arises when two compounds have the same molecular formula but differ in structure i.e., arrangement of atoms. (b) $\underset{\vert_{\mathrm{H}}}{\overset{\vert^{\mathrm{D}}}{\mathrm{C}}}=\underset{\vert_{\mathrm{D}}}{\overset{\vert^{\mathrm{H}}}{\mathrm{C}}}$ and $\underset{\vert_{\mathrm{H}}}{\overset{\vert^{\mathrm{D}}}{\mathrm{C}}}=\underset{\vert_{\mathrm{H}}}{\overset{\vert^{\mathrm{D}}}{\mathrm{C}}}$ are geometrical isomers. (c) $\mathrm{H}-\overset{,^{+}}{\overset{\parallel}{\mathrm{C}}\mathrm{OH}}-\mathrm{OH}$ and $\mathrm{H}-\mathrm{C}^{\overset{\vert^{\mathrm{OH}}}{+}}-\mathrm{OH}$ are resonance stabilized structures. Such structures arise when the arrangement of atoms remains the same but the bonding pattern differs.