The question asks about the speed at which a Squirrel cage induction generator operates for electric generation in wind turbines. Let's analyze the possible options step-by-step:

1. Option: Variable
   • A variable speed is typical for some types of modern wind turbine systems, especially those equipped with advanced power electronics that can handle frequency conversion. However, this is less common for simpler systems like a squirrel cage induction generator without additional control mechanisms.
2. Option: Slip
   • Slip refers to the difference in speed between the synchronous speed and the rotor speed of an induction machine. While slip is a relevant concept in induction machines, the generator itself must operate around a constant speed to align closely with the grid frequency.
3. Option: Constant
   • Squirrel cage induction generators generally operate at a constant speed, which is determined by the grid frequency. This is because they directly connect to the grid and follow the grid’s constant frequency. Therefore, for electric generation in wind turbines where squirrel cage induction generators are used, especially in traditional setups, the speed is generally constant.
   • This is the correct answer for a typical setup without variable speed control technologies.
4. Option: Negligible
   • A speed described as negligible doesn't make practical sense in this context, as substantial speed is necessary for power generation.

Conclusion: Based on the nature of squirrel cage induction generators and their typical operating conditions, the correct answer is that such generators operate at a constant speed when used for electric generation in wind turbines. This aligns with how they match the grid frequency, necessitating a stable rotational speed to function effectively.