In which of the following molecules carbon atom marked with asterix (*) is a stereocentre or Chiral centre?

The correct option is the first option (Option 1), which depicts the molecule with a central carbon atom bonded to hydrogen ($\text{-H}$), chlorine ($\text{-Cl}$), bromine ($\text{-Br}$), and a hydroxyl group ($\text{-OH}$).

Step-by-Step Explanation:

1. Concept of a Chiral Centre / Stereocentre:
A carbon atom acts as a chiral centre (or asymmetric carbon) when it satisfies the following two conditions:
• It is ${\text{sp}}^3$ hybridized, meaning it has tetrahedral geometry with four single covalent bonds.
• It is directly bonded to four entirely different atoms or groups of atoms.

2. Detailed Analysis of Each Option:

• First Option (Option 1):
As observed in the first image, the carbon atom marked with an asterisk (*) is bonded to:
1. A hydrogen atom ($\text{-H}$)
2. A chlorine atom ($\text{-Cl}$)
3. A bromine atom ($\text{-Br}$)
4. A hydroxyl group ($\text{-OH}$)
Since all four attached groups are different, this carbon atom is a stereocentre (chiral centre).

• Second Option (Option 2):
As shown in the second image, the asterisk-marked carbon is bonded to:
1. Two hydrogen atoms ($\text{-H}$)
2. A methyl group (${\text{-CH}}_3$)
3. An ethyl group (${\text{-C}}_2{\text{H}}_5$)
Because two identical hydrogen atoms are attached to it, this carbon atom is achiral and is not a stereocentre.

• Third Option (Option 3):
As shown in the third image, the asterisk-marked carbon is bonded to:
1. A hydrogen atom ($\text{-H}$)
2. A hydroxyl group ($\text{-OH}$)
3. Two identical methyl groups (${\text{-CH}}_3$)
Because the two methyl groups are identical, this carbon atom does not have four different substituents and is not a stereocentre.

• Fourth Option (Option 4):
As shown in the fourth image, the asterisk-marked carbon is bonded to:
1. Two hydrogen atoms ($\text{-H}$)
2. A chlorine atom ($\text{-Cl}$)
3. A fluorine atom ($\text{-F}$)
Because it is bonded to two identical hydrogen atoms, it is not a stereocentre.

Therefore, only the molecule in the first option contains a marked carbon atom that is a stereocentre.