Let's start with what we know:
Area of regular octagon = 1/2 x perimeter x apothem
We know the apothem, so all that we need to find to fill in the above equation is the perimeter:
perimeter = 8 * 5.8 = 46.4cm
Now we can fill in our original equation and solve:
Area of regular octagon = 1/2 x perimeter x apothem
Area = 1/2 x 46.4cm x 7cm = 162.4cm^2
The answer is B. 162.4cm^2