The mother of ‘the Little Black Boy’ says his dark skin and face are—

The correct answer is a cloud.

In William Blake's famous poem "The Little Black Boy" (from Songs of Innocence), the mother comforts her child by explaining the spiritual meaning behind their skin color. She teaches him that the body and dark skin are merely temporary coverings—described metaphorically as "a cloud" and "a shady grove"—that shelter human souls while they learn to receive God's love and light on Earth.

Specifically, the mother says:
"And we are put on earth a little space,
That we may learn to bear the beams of love;
And these black bodies and this sunburnt face
Is but a cloud, and like a shady grove."

Thus, she explains that his dark skin and face are like a cloud that will vanish once they transition to eternal life in heaven, making "a cloud" the correct choice.