Boundaries between intervals of the geological time scale are determined by
a. Major events in the Earth's history, such as major extinctions.
b. dividing the time range of the interval by the number desired subdivisions, thereby creating equal intervals between the divisions. c. Climatic periods based on paleoclimatological reconstructions.

Option (A)

Explanation:

The geological time scale is usually defined as the sequential arrangement of all the major and contrasting events that has taken place on earth right after the formation of the earth about 4.6 billion years back.

This scale is comprised of distinct eons, epochs and periods, which are classified on the basis of the major events occurred in the geological time, such as the great mass extinction events, asteroid impacts, abundant supply of oxygen, introduction of new and extinction of old living species.

This is how the boundaries between the intervals of the geological time scale are determine.

Thus, the correct answer is option (A).