What is the value of Joule-Thomson co-efficient for an ideal gas?

Answer: Option C

The joule Thomson coefficient is given as $${\mu }_i={\left(\frac{\partial T}{\partial P}\right)}_H,$$    And since for an ideal gas enthalpy is strictly only function of temperature which implies constant temperature and hence the joule thomson coefficient becomes zero. Physically both the contervining effects of throttling are balancing each other.