With which of the following reactants does the treatment of primary alcohol yield aldehydes?

Among the following compounds,the one that can be dehydrated most easily is:

What is the major product of the following reaction?
$CH_3-CH(OH)-CH_2-CH_2-OH \xrightarrow[Pyridine, \text{cold}]{CrO_3} \text{Product}$

The exact order of boiling points of the compounds $n$-pentane, isopentane, butanone, and $1$-butanol is:

The major product obtained by the dehydration of the given alcohol is $:-$

In the reaction sequence:
$\text{Ethanol}$ $\xrightarrow{PCl_5} X$ $\xrightarrow{\text{alc. } KOH} Y$ $\xrightarrow[H_2O, \Delta]{H_2SO_4, \text{room temp.}} Z$
The product $Z$ is: