A letter to the Editor is a _________ letter.

The correct answer is Option (3) → Formal

A letter to the editor is written to the editor of a newspaper or magazine, usually to raise a public issue. It is addressed to a person in an official position whom the writer does not know personally, so it follows the formal layout and tone: sender's address, date, editor's address, subject line, salutation "Sir/Madam", and "Yours truly/faithfully".
Example: She wrote a formal letter to the editor about the poor state of the roads.

Why the Other Options are Incorrect?

Option 1: Informal — Incorrect. An informal letter is written to family and friends, in a friendly, casual tone.

Option 2: Non formal — Incorrect. It is not a recognised kind of letter; letters are classed as formal or informal.

Option 4: Personal — Incorrect. A personal letter deals with private matters between people who know each other; a letter to the editor is meant for publication.