why dose the viceroy butterfly look like the monark butterfly?

Answer 1

Monarch Butterflies have a toxin so birds and other animals avoid eating them. Viceroys have evolved to resemble the Monarch. They are not toxic, but look like the Monarch, which is, so birds avoid them.

Answer 2

I might add to what the previous poster wrote: the monarch buttterfly caterpillar feeds on milkweed, which contains bitter-tasting chemicals, and these are incorporated into the monarch butterfly's body. The butterfly is not only toxic but it tastes foul to birds. So when a bird that has never encountered a monarch before tastes it, it usually spits the butterfly out (which saves the individual butterfly, if it hasn't been damaged too badly) and retains a memory of the butterfly as something that doesn't taste good. So it avoids monarchs-- and the similar-appearing Viceroy butterfly.

The biological term for this defense is "Batesian mimicry".

http://www.chebucto.ns.ca/Environment/NHR/monarch.html