Which of the following compounds has the maximum enol content?

Which acid of each pair shown here would you expect to be stronger?
$(i)$ $CH_3CO_2H$ or $CH_2FCO_2H$
$(ii)$ $CH_2FCO_2H$ or $CH_2ClCO_2H$
$(iii)$ $CH_2FCH_2CH_2CO_2H$ or $CH_3CHFCH_2CO_2H$
$(iv)$ $F_3C-C_6H_4-COOH$ or $H_3C-C_6H_4-COOH$

The electromeric effect in organic compounds is a....

What is the electromeric effect?

Which of the following orders of rotation barrier about the $C=C$ bond,as indicated in the structures $(I)$,$(II)$,and $(III)$,is correct?

The functional group that shows negative resonance effect ($-R$ effect) is: