I am learning acidity strength in organic compounds. Can you help me with this question.

Which of the phenol derivative is most acidic ?

It is based on how many electron donating or electron withdrawing groups present on the ring .EDG  decreases the acidity as they pull electrons from acidic H and EDG increases the acidity  as it donates electron density on the acidic H.
Compare electronegativities  of  halogen atoms  and try to figure out the answer.

Ohh.. you have made it so simple.
Electronegativity order of halogens is F>Cl>Br>I
It means most acidic is with flourine and least acidic is with I.

Perfect Rohit