Final answer:
In geology, the abbreviation 'a' stands for 'annum', which means 'years'. It is used to simplify the discussion of extensive geologic time periods, helping to describe events in Earth's history, such as those outlined on the geologic time scale.
Step-by-step explanation:
In geology, the term 'annum' is represented by the symbol 'a' and is used to replace the word 'years'. So, option 1) annum is the correct answer. When geologists discuss time periods in Earth's history, they often use abbreviations for practical purposes. For instance, 'ka' means thousands of years, while 'Ma' stands for millions of years, and 'Ga' indicates billions of years. These terms are crucial for discussing events on the geologic time scale, which is used to organize the vast history of Earth into more manageable periods. This scale includes divisions for Eons, Eras, Periods, Epochs, and Ages and is based on significant geological, biological, and climatic events, often determined through radiometric dating. Understanding these abbreviations is essential for interpreting geological time and the history of the Earth.