Answer:
The order is Star 3 < Star 2 < Star 1 < Star 4
Step-by-step explanation:
Lets name the stars with their written order.
Star 1: 2.7 x 10^18 km3
Star 2: 6.9 x 10^12 km3
Star 3: 2.2 x 10^12 km3
Star 4: 4.9 x 10^21 km3
Star with lowest power of 10 has the least volume. Therefore, star 2 and star 3 would be the stars with least volume. Star 2's coefficient is better than Star 3. Thus, the order will be Star 3 < Star 2 < Star 1 < Star 4.