1. Target product identification

We need to obtain $2,2$-dibromopropane.

Its structure is: $CH_3-CBr_2-CH_3$ This is a geminal dibromide with both bromine atoms on the middle carbon of propane.

2. Check each option

Option A

$CH_3-CH=CH_2 + HBr$ This is addition of one mole of HBr to propene.

By Markovnikov addition: $CH_3-CH=CH_2 + HBr \rightarrow CH_3-CHBr-CH_3$ Product is 2-bromopropane, not $2,2$-dibromopropane.

So, A is incorrect.

Option B

$CH_3-C\equiv CH + 2HBr$ This is propyne reacting with two moles of HBr.

First addition of HBr:

For terminal alkyne, Markovnikov addition gives: $CH_3-C\equiv CH + HBr \rightarrow CH_3-C(Br)=CH_2$

Second addition of HBr:

Again Markovnikov addition on the alkene gives: $CH_3-C(Br)=CH_2 + HBr \rightarrow CH_3-CBr_2-CH_3$

This is exactly $2,2$-dibromopropane.

So, B is correct.

Option C

$CH_3CH=CHBr + HBr$ Here the alkene is already brominated.

Adding HBr across the double bond can give a dibromopropane, but the product would be:

• bromine already on terminal carbon,
• incoming bromine adds according to carbocation stability.

The major product is: $CH_3-CHBr-CH_2Br$ which is 1,2-dibromopropane, not $2,2$-dibromopropane.

So, C is incorrect.

Option D

$CH\equiv CH + 2HBr$ Acetylene with two moles of HBr gives:

First addition: $CH\equiv CH + HBr \rightarrow CH_2=CHBr$

Second addition: $CH_2=CHBr + HBr \rightarrow CH_3-CHBr_2$

This is $1,1$-dibromoethane, not $2,2$-dibromopropane.

So, D is incorrect.

3. Conclusion

Only Option B yields $2,2$-dibromopropane.

$\boxed{B}$

4. Comparison with stored correct answer

Stored correct answer: B

My derived answer: B

They match.