Final answer:
The diamond is the birthstone for the month of April. Birthstones have historical significance and each month has its own gem with associated meanings.
Step-by-step explanation:
The birthstone for the month of April is a diamond. Therefore, the correct answer to the question 'What month's birthstone is a diamond?' is option b) April. Birthstones have a long history and are often associated with good luck and protection for the people born in that respective month. The tradition of birthstones can be traced back to ancient times, and each month has its unique gemstone with specific meanings and qualities attributed to it.