Which one is the correct declaration for choosing the character set other than default?

Answer: Option A

This question is about how to tell MySQL to use a specific character set for your data when creating a column.
Let's break down the options:
Option A: Varchar(20) character set utf8;
This is the correct way! It tells MySQL to use the utf8 character set for a varchar column that can hold up to 20 characters.
Option B: Varchar(20);
This option only defines the column as a varchar with a maximum length of 20 characters. It doesn't specify a character set, so it will use the default character set for your database.
Option C: Varchar(20) character set;
This is incorrect because it's missing the actual character set name. It's like saying "I want a car, but I didn't tell you what kind!"
Option D: None of the mentioned
This is incorrect because we already found the correct answer in Option A.
In summary, Option A is the only way to explicitly choose a character set other than the default one.