The correct answer is A. 1.4.
The shape factor is a dimensionless number that is used to determine the plastic collapse load of a structural member. It is defined as the ratio of the plastic moment of inertia of the cross-section to the elastic moment of inertia of the cross-section.
For a rectangular section, the shape factor is given by:
$S_f = \frac{I_p}{I_e} = \frac{bh^3}{6} \cdot \frac{1}{bh^3/12} = 1.4$
where $b$ is the width of the section, $h$ is the height of the section, and $I_p$ and $I_e$ are the plastic moment of inertia and the elastic moment of inertia of the section, respectively.
The shape factor is important because it determines the load-carrying capacity of a structural member. A higher shape factor indicates a higher load-carrying capacity.
The other options are incorrect because they are not the shape factor for a rectangular section.