H3O+ is nothing but the hydronium ion that is cation of the hydrogen atom.

The given concentration of H+ ion is 0.0001M

That is equal to 10^-4M

As we know, pH= -log[H+]

so putting values pH =-log[10^-4]

=-[-4]

=4

So pH is 4.

Practice more sums from the chapter Chemical Equilibrium of physical chemistry as this is a type of question that is frequently asked in entrance exams.

Best wishes. Thank you.