IUPAC name, phthalic anhydride
Benzofuran is named by the position of the oxygen, hence 2-Benzofuran.  (below is 1-benzofuran)



The =O s are then added on the 1 and 3 carbons, hence, 1-3-dione  

Compare, cyclopentan-1,3-dione 



The end result is 

2-benzofuran-1,3-dione