Alkanes and alkenes can be prepared by (a) Wurtz reaction (b) Williamson synthesis (c) Dehydrohalogenation (d) Kolbe's electrolysis

Before we classify each reaction as suitable for alkanes or alkenes preparation, we need to know what they are and the products they form: (a) Wurtz reaction: In this reaction, two alkyl halides are reacted with Sodium metal in a dry ether solvent to form a higher alkane. This reaction is used for preparing alkanes. (b) Williamson synthesis: It is an ether synthesis reaction. In this reaction, an alkyl halide reacts with an alcohol (or phenol) in the presence of a base to form an ether. This reaction is not used for preparing alkanes or alkenes. (c) Dehydrohalogenation: It is a reaction in which the alkyl halide loses a hydrogen atom and a halogen atom to form an alkene. This reaction is used for preparing alkenes. (d) Kolbe's electrolysis: In this reaction, salts of carboxylic acids are subjected to electrolysis, resulting in the formation of alkanes, alkenes, and other products.

Now that we have a clear understanding of each reaction, we can classify them based on whether they form alkanes or alkenes. (a) Wurtz reaction: As this reaction results in the formation of alkanes, it can be used for the preparation of alkanes. (b) Williamson synthesis: This reaction forms ethers, so it cannot be used for the preparation of alkanes or alkenes. (c) Dehydrohalogenation: This reaction results in the formation of alkenes, so it can be used for the preparation of alkenes. (d) Kolbe's electrolysis: As this reaction can result in the formation of both alkanes and alkenes, it can be used for the preparation of alkanes and alkenes. In conclusion, alkanes can be prepared by the Wurtz reaction and Kolbe's electrolysis. Alkenes can be prepared by Dehydrohalogenation and Kolbe's electrolysis. Williamson synthesis is not suitable for the preparation of either alkanes or alkenes.