Final answer:
The number of stars in the observed universe is approximately d. 10^20.
Step-by-step explanation:
To find the number of stars in the observed universe, we can multiply the number of stars in each galaxy by the number of galaxies. Given that each galaxy has 10^11 stars and there are 10^9 galaxies in the observed universe, we can calculate the total number of stars as follows:
Total number of stars = Number of stars per galaxy x Number of galaxies
Total number of stars = 10^11 x 10^9
Total number of stars = 10^20
Therefore, the number of stars in the observed universe is approximately 10^20 or option d) 10^20.