The major product formed in the reaction
`CH_(3)CH = CH - overset(O)overset(||)(C ) - CH_(3)underset(H_(2)O,CH_(3)OH)overset(NaBH_(4))rarr` is

Answer

To solve the problem, we need to analyze the reaction step by step:

1. **Identify the Reactants**: The reactant is 3-penten-2-one (CH3CH=CHC(=O)CH3), which contains a double bond and a ketone functional group.

2. **Identify the Reagent**: The reagent used is sodium borohydride (NaBH4), which is a mild reducing agent. It is known to reduce ketones to secondary alcohols and aldehydes to primary alcohols, but it does not reduce alkenes (double bonds).

3. **Reaction Mechanism**: Since we have a ketone (the carbonyl group C=O) in the molecule, NaBH4 will react with this carbonyl group. The double bond (C=C) will remain unaffected because NaBH4 does not reduce alkenes.

4. **Reduction of the Ketone**: The ketone group (C=O) will be reduced to a secondary alcohol (C-OH). The structure of the product will now include the unchanged double bond and the newly formed hydroxyl group (OH) from the reduction of the ketone.

5. **Final Structure of the Product**: After the reduction, the structure of the major product will be CH3CH=CHC(OH)CH3. This is 3-pentanol, where the ketone has been converted to a secondary alcohol.

6. **Conclusion**: The major product formed in the reaction is 3-pentanol.

### Final Answer:
The major product formed is **3-pentanol (CH3CH=CHC(OH)CH3)**.