These are the options available:
A. A long complex, math problem kept us busy for 30 minutes.
B. The sweet tangy scent of fresh limes surprised us.
C. Posters showed photographs of faraway, exotic places.
D. There was no sign of fish in the cold muddy waters.
The correct answer is C. Posters showed photographs of faraway, exotic places.
This is the only sentence that is punctuated correctly.
A is incorrect because there should be a comma between long and complex.
B is incorrect because there should be a comma between sweet and tangy.
D is incorrect because there should be a comma between cold and muddy.
You need to separate two or more adjectives when there are found next to each other.