Which isomerism is exhibited by the functional group diethylamine?

For the following four structures $I$ to $IV$,which statement is correct?
$(I)$ $C_2H_5-CH(CH_3)-C_3H_7$
$(II)$ $C_2H_5-CO-CH(CH_3)-C_2H_5$
$(III)$ $CH_3^+$ (Methyl carbocation)
$(IV)$ $C_2H_5-CH(CH_3)-C_2H_5$

Which of the following is an isomer of compound $(1)$?
$(1) \ CH_3CH_2CHO$
$(2) \ CH_3COCH_3$
$(3) \ CH_3CH=CHOH$
$(4) \ \text{2-methyloxirane (propylene oxide)}$

How many cyclic isomers are possible for ${C_5}{H_{10}}$?

Which of the following is a metamer of the given compound $(X)$?

The following are the structures of four isomers of hexane. Among the names given below,which correctly identifies the fifth isomer?
$CH_3CH_2CH_2CH_2CH_2CH_3$
$(CH_3)_3CCH_2CH_3$
$(CH_3)_2CHCH_2CH_2CH_3$
$(CH_3)_2CHCH(CH_3)_2$