The correct sentence is:
C. There is very little time left to sign up for dance lessons.
The other sentences contain errors:
A. Some people snore when their asleep.
The correct word should be "they're" instead of "their." The sentence should be: Some people snore when they're asleep.
B. For Australians, wheat is one of they're main crops.
The correct word should be "their" instead of "they're." The sentence should be: For Australians, wheat is one of their main crops.
D. ALL sentences are written correctly.
This statement is incorrect because sentences A and B have errors.