Optically active was found to have lost its optical activity after standing in water containing a few drops of acid, mainly due to the formation of 

So,the correct answer is A 

Optically active It was discovered that after standing in water containing a few drops of acid, it lost its optical activity, owing primarily to the formation of ${\mathrm{CH}}_3-\mathrm{CH}=\mathrm{CH}-{\mathrm{CH}}_3.$

Dehydration occurs to form stable alkene via stable carbocation formation. The more substituted the alkene, the more stable it is. 

The resulting alkene is acid catalyzed hydration to regenerate the original (but optically inactive) alcohol.