The correct answer is: C. 60 MW.
Multiple inlet system of air cooling of turbo-alternators can be used for machines of rating upto 60 MW. This is because the multiple inlet system provides a more efficient way to cool the alternator, which is important for machines of this size. The multiple inlet system also allows for a more even distribution of air flow, which helps to prevent overheating.
The other options are incorrect because they are not the maximum rating for a multiple inlet system of air cooling of turbo-alternators. Option A is incorrect because the maximum rating for a multiple inlet system of air cooling of turbo-alternators is 60 MW, not 10 MW. Option B is incorrect because the maximum rating for a multiple inlet system of air cooling of turbo-alternators is 60 MW, not 30 MW. Option D is incorrect because the maximum rating for a multiple inlet system of air cooling of turbo-alternators is 60 MW, not 150 MW.