After filing for extension and submitting, I am trying to pay estimated taxes due for 2024 on IRS.GOV website with credit card. What options do select in dropdowns?

What are your options in the dropdown?  You can pay your tax balance due, estimated payments or part of a payment plan with this IRS link >>  

IRS Payments website. 

Trying to choose what form from a dropdown menu with several choices. I'm assuming it is either 1040 series or form 4868

It would be Form 1040. You are paying taxes for your Form 1040, US Individual Income Tax Return.