Given, width, w=7 ft.
Area, A=84 square feet.
The area of rectangle is given by,
[tex]A=l\times w[/tex]Here, l is the length of the rectangle.
Put w=7 and A=84 to find l.
[tex]\begin{gathered} 84=l\times7 \\ \frac{84}{7}=l \\ 12feet=l \end{gathered}[/tex]Therefore, the length of the rectangle is 12 feet.
Hence, option B is correct.