To an astronaut in a spaceship, the colour of earth appears
(a) Red
(b) blue
(c) white
(d) black

Before knowing that first of all u shud know why the sky looks blue from earth. This is due to difrraction of light by the atmosphere. The blue light is having less wavelength than any other color of visible spectrum, so it diffracts very much in the atmosphere making it to appear blue. In space there is no atmosphere, so there is no diffraction and so the sky is black.

Talking about the color of the earth, the earth looks blue due to two reasons. First one is that earth consists mostly of water. And the second is Diffraction of light, because there is an atmosphere around earth and for the same reason mentioned above the blue light diffracts most while leaving the earth making it appear blue