PSD file

While @Jill_C  is correct. I would, however, also consider this: Except if your client is knowledgable about what the social media requirements are, you should provide them with correctly formated JPEG files, ready to use out of the box. Most customers can't do that correctly, when getting the source files. That's why they hire you. Also, you should keep a maximum of control over your work. It's at the end of the day your daily bread.

If the customer, however, requires transfer of the source files, do as Jill indicated. 

The format in which you supply designs to your client is between the 2 of you. 

Review the post pinned at the top of this forum to determine how to transfer the image license to your client.

https://community.adobe.com/t5/stock-discussions/transfer-license-to-a-client/td-p/11990240