what will be the alternative option after 2020 december 31st for flashplayer

Your main options are to:

1. Rebuild your applications using modern web technologies (HTML5, JavaScript, etc)
2. Migrate your web-based Flash applications to Adobe AIR desktop and/or mobile apps
3. Develop a custom browser with Flash Player support using Electron or similar, if it's an enterprise application used on your internal network (requires Flash Player distribution license)
4. Take a look at the impressive WebAssembly solution being developed by Leaning Technologies at https://www.leaningtech.com/pages/cheerpxflash.html

To be clear, if the site is not converted - and the site makers have had 3 years to do this - the site will stop working. Gone.

The alternative is for content providers to migrate their content from Flash Player to some other technology.