The formula for the relativistic length contraction is
$\begin{align*}L = L_0 \sqrt{1 - v^2/c^2}\end{align*}$
Plug in $L_0 = 4 \mbox{ m}$ and $v = 0.8 c$ (the result of the previous problem), to find that $L = 2.4 \mbox{ m}$ . Hence answer (A) is correct.

Solution to 1986 Problem 70