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A stereogenic center refers to a point in a molecule that can give rise to different isomers on the interchange of groups attached to it.

All the ${\text{sp}}^3$hybridized carbon atoms that are bonded to four non-identical/different groups are stereogenic centers.

The pink dots in the structure represent the stereogenic centre.

a. The given molecule consists of only one stereogenic center (carbon atom attached to four different groups).

Stereogenic centers in compound a

b. Only one stereogenic center is present in the given molecule as the molecule consists of only one carbon atom that is ${\text{sp}}^3$hybridized and is bonded to four different atoms.

Stereogenic centers in compound b

c. The given compound consists of two stereogenic centers as it consists of two ${\text{sp}}^3$hybridized carbon atoms bonded to four different groups.

Stereogenic centers in compound c

d. The given compound consists of two carbon atoms that are ${\text{sp}}^3$ hybridized and are attached to four different groups. Therefore, it contains two stereogenic centers.

Stereogenic centers in compound d

e. The given compound consists of six stereogenic centers.

Stereogenic centers in compound e

f. The given compound consists of four different ${\text{sp}}^3$ hybridized carbon atoms bonded to four different groups. Therefore, there are four stereogenic centers present in the molecule.

Stereogenic centers in compound f