Compound undergoes intramolecular nucleophilic substitution of the type `"………"` giving `"………"`

Which undergoes nucleophilic substitution most easily-

Which compound undergoes nucleophilic substitution with NaCN at the fastest rate ?

Which of the following compounds is the most likeyl to undergo a bimolecular nucleophilic substitution reaction with aqueous NaOH?

The compound which undergoes intramolecular dehydration with P_(2)O_(5) is