The correct option is B 12 square units
From the figure it is clear that, ∠CDB=∠ABD=90∘
Also, DC=AB=3 units (From figure)
Now, we know that a quadrilateral is a parallelogram if a pair of opposite sides is parallel and is of equal length.
∴ Quadrilateral ABCD is a parallelogram.
Now, area of the ||gm ABCD
=Base×Corresponding altitude
=AB×BD
=3×4 square units
=12 square units