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

Answer: Option A

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.