Should I choose a "Pass" grade instead of a letter grade due to the study disruption during the Covid-19?
You can choose “Pass” if you want but everyone will understand you made this choice to preserve your average, so your file will still look bad admission to a PhD program.
With “Pass” you may technically qualify to apply but I doubt admission into a PhD is simply a matter of meeting the technical requirements. Regardless of what you choose, you need to do better next term, so you can establish that this past term was just a one-off due to COVID.
On a related matter: are you sure you can still choose between “Pass” or a grade?  For most students the window to make that choice closed with or near the end of exams (at the latest).