The structure of 2-methylbutanal is CH3-CH2-CH(CH3)-CHO. The 2nd carbon atom (counting from the aldehyde group) is attached to different groups. so it is a chiral molecule.

Chiral Molecules:
Chiral molecules are molecules that are non-superimposable on their mirror image. This means that they exist in two different forms that are not identical, similar to how a left and right hand are not superimposable.

Explanation:
In order to determine whether a molecule is chiral or not, we need to examine its symmetry and the presence of chiral centers. A chiral center is an atom that is bonded to four different groups.

- a) 2-Butanone:
2-Butanone does not have a chiral center. It has a methyl group (CH3) and an ethyl group (CH3CH2) bonded to the carbon atom in question. The other two groups are both methyl groups (CH3). Since it does not have four different groups attached to the carbon atom, it is not chiral.

- b) Butanal:
Butanal also does not have a chiral center. It has a hydrogen atom, a methyl group (CH3), and an ethyl group (CH3CH2) bonded to the carbon atom in question. The fourth group is another hydrogen atom. Since it does not have four different groups attached to the carbon atom, it is not chiral.

- c) 2-Methylbutanal:
2-Methylbutanal, also known as isovaleraldehyde, does have a chiral center. It has a hydrogen atom, a methyl group (CH3), an ethyl group (CH3CH2), and a formyl group (CHO) bonded to the carbon atom in question. All four groups are different, making it a chiral molecule.

- d) 2-Methyl-1-propanal:
2-Methyl-1-propanal, also known as isobutyraldehyde, does not have a chiral center. It has a hydrogen atom, a methyl group (CH3), an ethyl group (CH3CH2), and another methyl group (CH3) bonded to the carbon atom in question. Since it does not have four different groups attached to the carbon atom, it is not chiral.

Therefore, the correct answer is option 'C', which is 2-Methylbutanal. It is the only molecule among the given options that has a chiral center.