The correct answer according to the map about the longest stretch of beach that is experiencing severe erosion is The coast between Houston and Mobile.
In this stretch we see only two tiny shoreline in orange between Houston and Mobile, which is the colour for moderately eroding and most of the shoreline in the stretch shows in red, which is the colour for severely eroding.
The second option The coast between Charlestone and Norfolk is wrong because many sections of the beach are shown in color orange or either in color white.
The third option is wrong because The northern coast of Alaska has many sections in orange too.
The last option is wrong because The western cost of California is almost in orange for moderately eroding.