In S_N1 (substitution nucleophilic unimolecular) reaction the racemization takes place. It is due to

both 1 and 2

The answer is (4) both 1 and 2, i.e., (1) inversion of configuration and (2) retention of configuration.

In an S_N1 reaction, the rate-determining step is the ionization of the alkyl halide to form a carbocation. The carbocation is a planar molecule, and the nucleophile can attack from either side of the plane, resulting in a mixture of products with inverted and retained configurations.

This is why S_N1 reactions are said to be racemizing reactions.

Option (1), inversion of configuration, is correct because the nucleophile can attack the carbocation from the opposite side of the plane, resulting in an inverted product.

Option (2), retention of configuration, is also correct because the nucleophile can attack the carbocation from the same side of the plane, resulting in a product with the same configuration as the starting material.

Option (3), conversion of configuration, is not correct because it is not possible for the nucleophile to convert the configuration of the carbocation.

Therefore, the racemization that takes place in S_N1 reactions is due to both inversion and retention of configuration.