What is the characteristic group of a secondary alcohol?

What type of alcohol is crotonyl alcohol?

Write $IUPAC$ names of the following compounds:
$(i)$ $CH_3-CH(CH_3)-CH(OH)-C(CH_3)_2-CH_3$
$(ii)$ $CH_3-CH(OH)-CH_2-CH(OH)-CH(C_2H_5)-CH_2-CH_3$
$(iii)$ $CH_3-CH(OH)-CH(OH)-CH_3$
$(iv)$ $HO-CH_2-CH(OH)-CH_2-OH$
$(v)$ $2-Methylphenol$ (structure)
$(vi)$ $4-Methylphenol$ (structure)
$(vii)$ $2,5-Dimethylphenol$ (structure)
$(viii)$ $2,6-Dimethylphenol$ (structure)
$(ix)$ $CH_3-O-CH_2-CH(CH_3)-CH_3$
$(x)$ $C_6H_5-O-C_2H_5$
$(xi)$ $C_6H_5-O-C_7H_{15}(n-)$
$(xii)$ $CH_3-CH_2-O-CH(CH_3)-CH_2-CH_3$

The acid which contains both $-OH$ and $-COOH$ groups is

$Crotonyl$ alcohol is an example of

Which of the following structures is an example of a symmetrical or simple ether?