Which one of the following on heating gives an unsaturated acid?

The compound which reacts with Fehling solution is

What is the decreasing order of acidic strength for the following acids?
$[I]$ Methanoic acid
$[II]$ Ethanoic acid
$[III]$ Propanoic acid
$[IV]$ Butanoic acid

Which among the following is the strongest acid?

Consider the following reaction. The major product $(P)$ formed is:

Give the reactions of benzoic acid $(C_6H_5COOH)$ with the following:
$(i)$ Sodium metal
$(ii)$ Solution of sodium hydroxide $(NaOH)$
$(iii)$ Solution of sodium bicarbonate $(NaHCO_3)$
$(iv)$ $P_2O_5$ or conc. $H_2SO_4$ $+$ heat
$(v)$ $PCl_5$
$(vi)$ $PCl_3$
$(vii)$ $SOCl_2$
$(viii)$ $NH_3$ $+$ high temperature
$(ix)$ $LiAlH_4$ / ether or $B_2H_6$
$(x)$ Sodalime $(NaOH + CaO)$ $+$ heat
$(xi)$ Conc. $HNO_3$ $+$ conc. $H_2SO_4$ $+$ heat
$(xii)$ $Br_2 + FeBr_3$
$(xiii)$ Oleum $(H_2SO_4 + SO_3)$ $+$ heat