Hello ,i created Quick action on account and now i have to add on activites as shown in below how can we add ?

@Jyoti Bhosale the system decides where buttons are placed based on what the button does. The only buttons that will show up under the activity tab are activity-related buttons (events, tasks, emails, calls, or custom buttons that create activities)

All other buttons will show up in the top right-hand corner of the page.

This Trailhead explains it in more detail: https://trailhead.salesforce.com/en/content/learn/modules/lex_migration_customization/lex_migration_customization_actions

Hence.. you can add custom action as a Tab next to activity. That is possible