The correct answer is: A. only M
A meso compound is a stereoisomer that is optically inactive due to internal compensation. This means that the compound has two or more chiral centers, but the enantiomers cancel each other out, resulting in a molecule that does not rotate plane-polarized light.
In the case of compound M, there are two chiral centers (the two carbon atoms with four different groups attached). However, the two enantiomers of M are mirror images of each other, and they can interconvert through a rotation around the central carbon-carbon bond. This means that the two enantiomers of M are not actually separate molecules, but rather two different conformations of the same molecule. As a result, compound M is a meso compound.
In the case of compound K, there are also two chiral centers. However, the two enantiomers of K are not mirror images of each other. This means that the two enantiomers of K are actually separate molecules, and they cannot interconvert through a rotation around the central carbon-carbon bond. As a result, compound K is not a meso compound.
In the case of compound L, there is only one chiral center. This means that compound L cannot exist as a meso compound.
Therefore, the only molecule that exists as a meso structure is compound M.