Which reaction is known as the addition of dihydrogen to an alkene? Explain briefly.

(N/A) The addition of dihydrogen $(H_2)$ to an alkene is known as hydrogenation.
In this reaction,alkenes react with dihydrogen in the presence of finely divided catalysts like nickel $(Ni)$,palladium $(Pd)$,or platinum $(Pt)$ to form alkanes.
This is an example of a catalytic reduction reaction.
For example: $CH_2=CH_2 + H_2 \xrightarrow{Ni/Pd/Pt} CH_3-CH_3$ (Ethane).