Which among the following is the correct syntax for modifying the definition of an existing table?

This question is about changing the structure of an existing table in MySQL. Let's break down the options:
Option A: ALTER TABLE person MODIFY person_id SMALLINT UNSIGNED AUTO_INCREMENT;
This is the correct syntax. It uses the ALTER TABLE command to modify the table "person". The MODIFY keyword tells MySQL to change the "person_id" column. We are making it a SMALLINT (a smaller integer type), UNSIGNED (meaning only positive numbers), and AUTO_INCREMENT (meaning the database automatically increases the value for each new row).
Option B: ALTER TABLE person person_id SMALLINT UNSIGNED AUTO_INCREMENT;
This is incorrect. It's missing the MODIFY keyword, which is essential for making changes to a column's definition.
Option C: ALTER TABLE person MODIFY person_id ;
This is incorrect. It's missing the new data type and other attributes for the "person_id" column. You need to specify how you want to change the column.
Option D: ALTER TABLE person
This is incomplete. It only starts the ALTER TABLE command, but doesn't specify what changes you want to make.
Therefore, the correct answer is Option A.