Which of the following is an OXYMORON ?

The correct answer is Option (3) → The dull bright afternoon looked no different

Oxymoron:

• An oxymoron places two words of opposite meaning side by side.
• The clash is deliberate: together the two words describe something more sharply than either could alone.
• This sentence: dull (lacking light or interest) and bright (full of light) contradict each other, yet "dull bright afternoon" describes an afternoon that was sunny but uneventful.
  Example: The office was filled with a deafening silence after the news.

Why the Other Options are Incorrect?

Option 1: The night sky with all the stars was a map for the traveller — Incorrect. The sky is called a map directly, without "like" or "as". This is a metaphor, not a pair of opposite words.
Example: The classroom was a zoo when the teacher stepped out.

Option 2: Anita sang like a professional singer — Incorrect. Anita is compared to a professional singer with "like", so this is a simile.
Example: He ran like the wind.

Option 4: Amazing aunty Agatha opened the door — Incorrect. Three words in a row begin with the letter "a" (Amazing, aunty, Agatha). Repeating the same opening letter or sound in nearby words is alliteration.
Example: Peter Piper picked a peck of pickled peppers.

Contrast: an oxymoron sets two opposite words side by side ("dull bright"); a metaphor, a simile and alliteration join no opposites.