The correct answer is option 1: 20 aircrafts with 4 in reserve.
Option 1 states that the first Tejas squadron of the Indian Air Force consists of 20 aircrafts with 4 in reserve. This means that there are a total of 20 operational aircrafts that are ready for use, and an additional 4 aircrafts that are kept in reserve for backup or maintenance purposes.
Option 2, which states that the first Tejas squadron consists of 100 aircrafts with 10 in reserve, is incorrect. The correct number of aircrafts in the first Tejas squadron is 20, not 100.
Option 3, which states that the first Tejas squadron consists of 20 aircrafts with no reserve, is also incorrect. The correct answer is that there are 4 aircrafts in reserve.
Option 4, which states that the first Tejas squadron consists of 25 aircrafts with 5 in reserve, is also incorrect. The correct number of aircrafts in the first Tejas squadron is 20, not 25.
Therefore, the correct answer is option 1, 20 aircrafts with 4 in reserve.