Show as Conversations options
To enable or disable Show Conversations, switch to the View tab and check or uncheck the Show Conversations option. Expand Conversation Settings to fine-tune the conversation display.
Show as conversations is only available when you group by date. If it's grayed out, you need to switch to the Date grouping. When Show as conversations is enabled, the Date group is labeled Date (Conversations).
In the Arrangement group select Date. A dialog box will open, asking if you want to arrange by conversation in all folders or this folder only, as well as an option to cancel. This dialog box comes up each time you enable or disable Show as Conversations.