Which of the following plurals is spelled correctly?
ponys
diaries
studees
berryes

Diaries is spelled correctly. This is because if the singular version ends in a 'y', to make it plural, drop the 'y' and add 'ies'.

Here's an example.
The singular form of puppy is puppy.
Now we will drop the 'y'. Now we have pupp.
Now add 'ies'. Now we have puppies.
Hope I helped!

All of these wordss, in singular form, end with y

pony
diary
study
berry

there is a rule. Whenever a word end with y and you make it plural you usually replace y with ies

Which one is correct?

ponys
diaries
studees
berryes

there is only one answer that ends in ies
So the answer is diaries