Okay so,
According to the question, we know that the length (longer side) of the rectangle, which we can denote as the letter l, is 12 cm.
Also, it is said that the width of the rectangle (also called the breadth), which we can denote as w (or b), is one-third of its length. This basically means that if you cut up the length into 3 parts, the length of one part of it is how long the width is.
Therefore, in a simpler way, 12 * (1/3) = 4cm is the width of the rectangle.

We know that the area of a rectangle is given by the formula of
Area of rectangle (A) = length * width ( l * w )
Hence,
Area = 12 cm * 4 cm
Area = 48 cm^2 (Ans)

This option is not given in the question!

Hope you understood the solution.
In case of any queries make sure to reply to this thread.
Make sure to view the image attached for clearer understanding.

Happy Learning!😊