Final answer:
The number of possible outcomes when selecting two marbles from the bag without replacement is 6.
Step-by-step explanation:
The number of possible outcomes when selecting two marbles from the bag without replacement can be determined using the concept of combinations. In this case, we have four marbles in the bag (1 green, 1 blue, 1 red, and 1 yellow). The number of ways to select two marbles from four is given by the formula nCr, where n is the total number of marbles and r is the number of marbles being selected. Therefore, the number of possible outcomes is 4C2 = 6.