davmerit on "[Plugin: Contact Form 7] Contact form 7 Custom field as per dropdown"

Looks like you are looking for Conditional Logic which is not available directly in the Contact Form 7 plugin.

This sort of conditional logic is not available directly in Contact Form 7. It can be done via custom programing but you would need to be rather skilled at WordPress development using PHP and/or jQuery.

Some of the premium Form plugins do offer some Conditional Logic including Gravity Forms and Ninja Forms.

Also see https://wordpress.org/support/topic/plugin-contact-form-7-this-is-how-to-showhide-fields-with-jquery