`CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)C-CH=CH_(2)+HBr to A`
The structure of A is:

Options

A. `CH_(3)-underset(H_(3)C)underset(|) overset(H_(3)C)overset(|)C-underset(Br)underset(|)CH-CH_(3)`
B. `CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)C-CH_(2)-CH_(2)Br`
C. `CH_(3)-underset(H_(3)C) underset(|) overset(Br)overset(|)C-underset(CH_(3))underset(|)(CH)-CH_(3)`
D. None of these

Explanation

To determine the structure of compound A formed from the reaction of the given alkene with HBr, we will follow these steps:

### Step 1: Identify the Reactant
The reactant is a compound with the structure:
```
CH3
|
CH3 - C = C - CH2
```
This compound has a carbon-carbon double bond (alkene) and three methyl groups attached to the carbon chain.

### Step 2: Add HBr to the Alkene
When HBr is added to an alkene, the reaction follows Markovnikov's rule. This means that the hydrogen (H) from HBr will attach to the carbon atom that has more hydrogen atoms already attached (the less substituted carbon), and the bromine (Br) will attach to the other carbon atom (the more substituted carbon).

### Step 3: Protonation of the Alkene
The double bond attacks the hydrogen ion (H⁺) from HBr:
```
CH3
|
CH3 - C - C⁺ - CH2
```
This results in the formation of a carbocation. In this case, we have a secondary carbocation because the positively charged carbon is attached to two other carbon atoms.

### Step 4: Carbocation Rearrangement
Since we have a secondary carbocation, it is possible for rearrangement to occur to form a more stable tertiary carbocation. A methyl group can shift from an adjacent carbon to stabilize the positive charge:
```
CH3
|
CH3 - C - C - CH3
```
This results in a tertiary carbocation.

### Step 5: Nucleophilic Attack by Bromide Ion
The bromide ion (Br⁻) will now attack the positively charged carbon atom, leading to the formation of the product:
```
CH3
|
CH3 - C - C - Br
```
This gives us the final structure of compound A.

### Final Structure of A
The final structure of compound A can be represented as:
```
CH3
|
CH3 - C - C - Br
```
This is 2-bromo-2-methylbutane.

### Answer
The structure of A is 2-bromo-2-methylbutane.

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