What is the number of structural isomers of $C_3H_6O$?

What is chain isomerism? Explain by giving an example.

The starting material is propene $(CH_3-CH=CH_2)$. The reaction with $H_3O^+$ (acid-catalyzed hydration) follows Markovnikov's rule to give $A$ (propan$-2-$ol,$CH_3-CH(OH)-CH_3$). The reaction with $(i) BH_3/THF$ followed by $(ii) H_2O_2/OH^-$ (hydroboration-oxidation) follows anti-Markovnikov's rule to give $B$ (propan$-1-$ol,$CH_3-CH_2-CH_2OH$). What is the relationship between products $A$ and $B$?

Which among the given molecules can exhibit tautomerism?

The isomerism exhibited by alkyl cyanide $(R-CN)$ and alkyl isocyanide $(R-NC)$ is:

What is structural isomerism? Give two examples of chain isomerism in alkanes. Or,explain the isomerism of alkanes having more than three carbon atoms using two examples.