In which reaction product formation takes place by Saytzeff rule

Options

A. `CH_3-CH_2-overset(Br)overset(|)(CH)-CH_3overset(CH_3ONa//Delta)rarr`
B. `CH_3-underset(CH_3)underset(|)overset(OH)overset(|)C-CH_3overset(conc.H_2SO_4//Delta)rarr`
C. `CH_3-underset(CH_3)underset(|)overset(Br)overset(|)C-CH_3overset(Alc.KOH//Delta)rarr`
D. `CF_3-CHCl_2overset(AlC.KOH//Delta)rarr`

Answer

To determine in which reaction product formation takes place by Saytzeff's rule, we need to analyze the given options and identify the elimination reactions that lead to the formation of more substituted alkenes as per Saytzeff's rule.

### Step-by-Step Solution:

1. **Understanding Saytzeff's Rule**: Saytzeff's rule states that in an elimination reaction, the more substituted alkene will be the major product. This means that when a hydrogen atom is removed from a carbon atom adjacent to the carbon with the leaving group, the hydrogen should be removed from the carbon that leads to the more stable (more substituted) alkene.

2. **Analyzing the First Option**:
- The compound is CH3-CH2-CHBr-CH3.
- When elimination occurs (for example, using a base like CH3ONa), we can remove a hydrogen from either of the two adjacent carbons (CH2 or CH3).
- If we remove a hydrogen from the CH2 (the carbon next to the bromine), we get CH3-CH=CH-CH3 (butene).
- If we remove a hydrogen from the CH3, we get CH3-CH2-CH=CH2 (also butene).
- According to Saytzeff's rule, the more substituted product (CH3-CH=CH-CH3) will be favored because it is more stable due to having more alkyl groups attached to the double bond.

3. **Analyzing the Second Option**:
- The compound is CH3-CH2-CH(OH)-CH3.
- Here, when elimination occurs, we can remove a hydrogen from any of the CH3 or CH2 groups.
- Since all the adjacent carbons have the same number of hydrogen atoms, only one product will be formed (no preference for more substitution), hence no Saytzeff product.

4. **Analyzing the Third Option**:
- The compound is CH3-CH(Br)-CH2-CH3.
- Similar to the previous case, the removal of hydrogen can occur from either CH3 or CH2 groups.
- Again, since all adjacent carbons have the same number of hydrogen atoms, only one product will be formed, hence no Saytzeff product.

5. **Analyzing the Fourth Option**:
- The compound is CF3-CHCl2.
- In this case, there is only one hydrogen available for elimination.
- Thus, only one product can be formed, which does not allow for the application of Saytzeff's rule.

6. **Conclusion**: The only option where Saytzeff's rule applies is the first one (CH3-CH2-CHBr-CH3) because it allows for the formation of a more substituted alkene as the major product.

### Final Answer:
The reaction in which product formation takes place by Saytzeff's rule is **Option 1: CH3-CH2-CHBr-CH3**.