How to run only one task in ansible playbook?

You should use tags: as documented in https://docs.ansible.com/ansible/latest/user_guide/playbooks_tags.html

If you have a large playbook it may become useful to be able to run a specific part of the configuration without running the whole playbook.

Both plays and tasks support a “tags:” attribute for this reason.

Example:

tasks:

    - yum: name={{ item }} state=installed
      with_items:
         - httpd
         - memcached
      tags:
         - packages

    - template: src=templates/src.j2 dest=/etc/foo.conf
      tags:
         - configuration

If you wanted to just run the “configuration” and “packages” part of a very long playbook, you could do this:

ansible-playbook example.yml --tags "configuration,packages"

On the other hand, if you want to run a playbook without certain tasks, you could do this:

ansible-playbook example.yml --skip-tags "notification"

You may also apply tags to roles:

roles:
  - { role: webserver, port: 5000, tags: [ 'web', 'foo' ] }

And you may also tag basic include statements:

- include: foo.yml tags=web,foo

Both of these have the function of tagging every single task inside the include statement.