After the Mission: Priority Mars, you’ll be able to obtain War Assets from various activites including, Assignments, Missions, and helping random citizens throughout the game. Some can even be found by scanning planets. Doing these will increase your Effective Military Strength (EMS) which can make a major difference on the endings, which is shown below.
The final choice in Mass Effect 3 is determined by your Effective Military Strength (EMS), and the availability of the choices available depends on how high it was when you get to this point.
Less than 1,750 EMS
If you get to the end of the game with a score less than 1,750, you’ll only get one option in the final choice with the Crucible. This choice will either be Control or Destroy, and the choice you’ll have from them will be dependent on whether you destroyed the Collector Base or not in Mass Effect 2.
In between 1,750 - 2,649 EMS
If you get to the end of the game with a score in-between 1,750 - 2,649 EMS, you’ll get two options in the final choice with the Crucible. These will be either Control or Destroy regardless of your choice in Mass Effect 2, and choosing Destroy will no longer end all life on Earth.
In between 2,800 - 3,099 EMS
If you get to the end of the game with a score in-between 2,800 - 3,099 EMS, you’ll get three options in the final choice with the Crucible. Sythesis will be the new choice added, and with your EMS this high, none of the choices will end up with Earth being ravaged, and the Mass Relays won’t be destroyed.
3,100 EMS or Higher
If you get to the end of the game with a score of 3,100 EMS or Higher, you’ll get all three options again in the final choice with the Crucible. Also, you’ll be able to survive on the Destroy ending, even if Anderson is killed by the Illusive Man.