The correct option is A It's about the existence of God.
The poet, Emily Dickinson, says that though she has never seen a moor or the sea in her life, she can imagine what it looks like. In the same way, she has never spoken to God nor been to Heaven, but she knows how she can go to Heaven (by being religious).
Option A: The poem sends a message of faith in God, by first mentioning the moor and the sea, and then speaking about God. Through the poem, she says that despite the fact that she hasn't seen or spoken to God, she knows that he exists.
Hence, option A is correct.
Option B: The poet does not focus on heaven but on God. She says that she will go to heaven even though she has never met God. So, 'heaven and earth' are metaphors to convey her message (her belief in the existence of God), not the message itself.
Options C and D: The poem neither mentions nor implies these things.
Hence options B, C and D are incorrect.