Explanation:

Chirality:
Chirality is a property of certain molecules that are non-superimposable mirror images of each other. These molecules are called enantiomers. Chiral molecules have a special type of symmetry known as "handedness." They cannot be superimposed on their mirror images, just like our left and right hands.

Enantiomers:
Enantiomers are a pair of molecules that are non-superimposable mirror images of each other. They have the same physical and chemical properties except for their optical activity. Each enantiomer rotates the plane of polarized light in equal but opposite directions. One enantiomer rotates the light to the right, while the other rotates it to the left. Enantiomers have opposite configurations at all chiral centers.

Racemic Mixture:
A racemic mixture is a mixture that contains equal amounts of both enantiomers. Since these enantiomers have opposite configurations at chiral centers, their rotations cancel each other out, resulting in zero net optical rotation.

SN1 Reaction:
SN1 reactions proceed through a two-step mechanism. In the first step, the leaving group leaves, forming a carbocation intermediate. In the second step, the nucleophile attacks the carbocation, resulting in the formation of the product. SN1 reactions do not involve any stereochemistry, so they do not result in the formation of a specific configuration at the reactive site.

SN2 Reaction:
SN2 reactions proceed through a single-step mechanism in which the nucleophile attacks the substrate while the leaving group departs. In an SN2 reaction, the configuration at the reactive site is inverted. This is known as "Walden inversion." The nucleophile attacks from the opposite side of the leaving group, leading to the inversion of configuration.

Incorrect Statement:
The incorrect statement regarding chirality is option 'B' - "Enantiomers are superimposable mirror images of each other." This statement is incorrect because enantiomers are non-superimposable mirror images of each other. They have the same physical and chemical properties, but they are different in their optical activity and have opposite configurations at all chiral centers.

Correct Statements:
a) The product obtained by SN2 reaction of a haloalkane having chirality at the reactive site shows inversion of configuration.
c) A racemic mixture shows zero optical rotation.
d) SN1 reaction yields a 1:1 mixture of both enantiomers.

Summary:
Enantiomers are non-superimposable mirror images of each other, and each enantiomer has opposite configurations at chiral centers. A racemic mixture shows zero optical rotation, and SN1 reactions yield a 1:1 mixture of both enantiomers. In contrast, the product obtained by an SN2 reaction of a chiral haloalkane shows inversion of configuration at the reactive site.