Hey there!
Your answer is a. white poster board.
The color an object appears as depends on what colors of light are reflected vs absorbed.
If an object appears as black, that means it doesn't reflect light, but it absorbs the light.
If an object appears as white, that means it doesn't absorb light, but it reflects the light.
White light is all colors of light mixed together.
So, the white poster board is the object reflecting almost all the light that strikes it.
Hope this helps!