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Step-by-Step Solution Step 1: Identify the Reaction Context The question asks us to determine the predominantly formed product (“A”) from the given set of possible structures. Typically, when an unsymmetrical reagent or an unsymmetrical alkene/alkyne is involved, the major product is decided by various factors such as the stability of the intermediate (carbocation, free radical), Markovnikov’s rule, or possible rearrangements. Step 2: Analyze the Possible Products All four options represent different structural isomers or rearranged derivatives. In an electrophilic addition reaction, the pathway that leads to the most stable intermediate will form the major product. If rearrangement to a more stable carbocation is possible, it often dominates, giving the rearranged product as the major outcome. Step 3: Determine the Most Stable Intermediate Whenever a carbocation intermediate forms (for example, in the addition of $HX$ to an alkene), the reaction pathway that yields the more stable carbocation (tertiary > secondary > primary) will prevail. If a hydride or methyl shift can produce a more stable carbocation, that rearrangement usually occurs. Step 4: Conclude the Predominant Product Based on such rearrangement or stability considerations (seen by checking or recalling standard reaction mechanisms), the correct answer is the structure shown in Option 4. This choice arises from either Markovnikov addition (if no rearrangement is needed) or from a rearrangement that generates the most stable carbocation and hence the most stable final product. Step 5: Final Answer Verification The solution (also shown below) confirms why the predominantly formed product aligns with structure 4. Always verify by comparing predicted major product from mechanistic principles and rearrangement rules to ensure correctness. Solution Reference Diagram

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