Answer is E
As In Spelling of Earth E comes first, in Heaven it come s second, in Week it comes twice, in a year it come once and finally in spelling of sea E stands in middle of the spelling.
References:http://puzzle.queryhome.com/1148/guess-me-who-am-i-i-am-the-first-on-earth-the-second-in-heaven?show=1168#a1168