Choose the correct option :

In a formal letter we write _________ sincerely.

yours

The correct answer is Option (3) - yours

In a formal letter, the correct closing phrase is:

 "Yours sincerely," (when addressing someone by name)
  "Yours faithfully," (when addressing someone without a specific name, e.g., "Dear Sir/Madam")

OTHER OPTIONS

 "Your" → Incorrect

• "Your" is a possessive adjective (e.g., Your book is on the table), but it is not used in letter closings.

 "your's" → Incorrect

• The correct spelling is "yours".
• "Your's" is grammatically incorrect because possessive pronouns (e.g., yours, ours, theirs) do not take an apostrophe.

 "you" → Incorrect

• "You" is a pronoun and does not fit in this context.