Q: In this task, you need to answer the given multiple-choice question on geometry. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.
Problem: the diagonal of a square is 40 m . the area of the square is : 
Options: ['a ) 600', 'b ) 900', 'c ) 800', 'd ) 500', 'e ) 400']
A:
c