The 20th century is the most diverse classical music period.

Carnatic music.

The word 'classical', when applied to music, has two different meanings, a narrow one and a much broader one.

In the narrow sense of the word the classical period in music extended roughly from the middle of the eighteenth century to the early nineteenth century.

Classical music in the broader sense of the expression includes the 'serious music' or 'art music' of the last four centuries (or perhaps longer), continuing today.

That depends, the classical period was from the early-mid 1700's to the early 1800's, Classical Music as a whole is from about 1680-now

Classical music (as we know it today) has existed since the 9th century and into the present day. The term "classical music" wasn't applied formally until the 19th century.