Can anyone tell me what reaction is this?
Diazomethane under the action of light or heat releases nitrogen leaving a  neutral, divalent carbon species with a lone pair and an empty orbital called a carbene. This carbene then reacts with the double bond of alkene breaking it to form a cyclo- species. in your example, cis-2-butene will react with diazonmethane to form cis-1,2-dimethylcyclopropane with no detectable amount of the trans isomer.

However, do note that if the same reaction is carried out via a triplet carbene, generated via triplet sensitization, then a 1.3:1 mixture of trans to cis dimethylcyclopropane is observed. [3]





Source


https://www.masterorganicchemistry.com/2011/11/05/reagent-friday-diazomethane-ch2n2/
[Source for pic. 1]
http://www.ochempal.org/index.php/alphabetical/c-d/diazomethane/ [Source for pic. 2]
http://reag.paperplane.io/00000832.htm
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Alkenes/Reactivity_of_Alkenes/Diazomethane%2C_Carbenes%2C_and_Cyclopropane_Synthesis