The correct answer is option 4: 1 and 2 only.

Statement 1 is correct. The magnetic field strength in a solenoid depends upon the number of turns per unit length in the solenoid. In simpler terms, the more turns there are in a solenoid, the stronger the magnetic field it produces. This is because each turn contributes to the overall magnetic field.

Statement 2 is also correct. The magnetic field strength in a solenoid depends upon the current flowing in the wire of the solenoid. Increasing the current increases the magnetic field strength. This is because the magnetic field produced by an electric current is directly proportional to the magnitude of the current.

Statement 3 is incorrect. The magnetic field strength in a solenoid does not depend on the diameter of the solenoid. The diameter of the solenoid only affects the shape and size of the magnetic field but not its strength.

In summary, the correct statements are that the magnetic field strength in a solenoid depends on the number of turns per unit length and the current flowing in the wire of the solenoid. The diameter of the solenoid, however, does not affect the magnetic field strength.