Question

The D in the given reaction is:

1) But-2-ene 2)1-butyne 3)Butane 4)Butanal

Answer 1

This is hydroboration oxidation reaction. A molecule of water is added to the carbon carbon double bond.
Since borane contains deuterium instead of hydrogen, HOD is added to the double bond instead of HOH. Thus -OH group and a D atom are attached to the carbon atoms joined by double bond.