What do the solid wedge and dashed wedge symbols indicate in the 3D representation of an organic molecule?

The least acidic compound among the following is:

An organic compound $C_3H_6O$ does not give a precipitate with $2,4-$Dinitrophenylhydrazine reagent and does not react with metallic sodium. The compound is:

State whether the following statements are True or False:
$(i)$ There is a huge number of organic compounds because the carbon atom is very small.
$(ii)$ The number of organic compounds is huge because of catenation.
$(iii)$ Organic compounds are in a huge number because of valency.

The exact order of acidity of the compounds $p-$nitrophenol, acetic acid, acetylene and ethanol is

On mixing tetraethyl lead to gasoline available at petrol pumps,