Final answer:
Approximately 50% of over 30 known STIs are incurable.
Step-by-step explanation:
The correct answer is d) 50%.
According to the information provided, there are now over 30 known STIs (Sexually Transmitted Infections) and approximately 50% of them are incurable.
It is important to note that the percentage of incurable STIs can vary depending on the specific time and location, as new infections and treatments may change these numbers over time.
Hence, Approximately 50% of over 30 known STIs are incurable.