The correct answer is C. Rigorous.
A stringent rule or requirement is one that is very strict and demanding. It is often used in the context of law or education, but can also be applied to other areas of life. For example, a stringent dress code might require students to wear a uniform, while a stringent test might be very difficult.
The other options are not as accurate descriptions of a stringent rule or requirement. A dry rule or requirement might be boring or uninteresting, but it would not necessarily be strict or demanding. A strained rule or requirement might be difficult to follow, but it would not necessarily be strict or demanding. A shrill rule or requirement might be loud or piercing, but it would not necessarily be strict or demanding.
In conclusion, the correct answer is C. Rigorous. A stringent rule or requirement is one that is very strict and demanding.