We are working on a sharepoint 2013 team site. and inside the document library we have a .xls file which contain 5 tabs. so now oor users are asking if multiple users can work simultaneously on the same .xls file tabs ? Now based on the available options within the document library i did not find any thing related to this.
This depends a bit on the setup.
The filetype you mentioned is not ideal. You should check and upgrade to xlsx.
Besides that it depends on your Version of Excel. WAC (Word/Excel Online) allows you to coauthor and also the latest Versions should allow it. So if you have an office web app farm with your SharePoint it should work. If not you would need a current excel Version. Since you mention SP2013 I put current in bold ;)
A bit more information can be found here:
They keyword to search for more info would be co-authoring.
 One of the key points is that not all versions of office support coauthoring of excel files. For SP2013 you need the office WAC server
Here is an overview link: https://docs.microsoft.com/en-us/SharePoint/governance/co-authoring-overview
Audience: IT Professionals Use the co-authoring feature in SharePoint Server or SharePoint Online to enable multiple users to work on a document, at any time, without interfering with each other's changes. Co-authoring removes barriers to server-based document collaboration and helps organizations to reduce the overhead associated with traditional document sharing through attachments. This functionality requires no additional server setup and is the default state for documents stored in SharePoint Server and SharePoint Online. Co-authoring functionality is managed by using the same tools and technologies that are already used to manage SharePoint, helping to minimize the impact on administrators. Office provides co-authoring functionality for Word, PowerPoint, OneNote, and Visio. If you are using SharePoint Online or have SharePoint Server configured to use Office Web Apps Server, users can also co-author documents in Word, PowerPoint, Excel, and OneNote Web Apps.
Hope that helps