Chemistry - Carbonyl compounds Question with Solution | TestHub

Diisobutylaluminium hydride, $\mathrm{AlH}(\mathrm{i}-\mathrm{Bu}{)}_2$  also known as DIBAL or DIBAL-H, is an exceedingly useful and versatile reducing agent. It is an electrophilic reducing agent, usually employed in selective reductions of esters or nitriles to aldehydes

So, in first step nitrile group will be reduced to aldehyde group. 

In second step cross aldol condensation reaction will take place which will give mixture of products. Out of which one of the product will be option B.