18) E

This requires just listing all the possible arrangements.

When 4 yellow and 2 red skittles are used,
There are 15 possible arrangements.

When 4 red and 2 yellow skittles are used,
There are 15 possible arrangements.

When 3 yellow and 3 red skittles are used,
There are 20 possible arrangements.

In total, there are 50 possible arrangements.