out of phenol,meta ortho and para nitro phenols which is most acidic and why?

The order will be

Meta > Para > Ortho

This is because of the following reasons

Meta is the strongest acid due to greater -I effect and weak hydrogen bonding.

Moving to para , here though the intramolecular hydraogen bonging are quite weak as compared to meta but so is the -I effect which results in decrease in acidity, So it is less acidic than meta but more than ortho

Now coming ortho, it is least acidic due to strongest hydrogen bonding. In this case the hydrogen bonding is so strong that even the -I effect cannot compensate for this