Hello,
This solution can be developed in C#.Net and will be using the nuget whatsapp library (web version) for sending and receiving messages. The Application will be fully responsive will be using Async calls so that the it will notify user of any sent/received messages with out waiting. The application can have desktop or web presentation whatever suits the requirements.
I have extensive experience in .Net (C#), working in the software development industry for more than five years i have worked on developing complex applications in almost all areas e.g. Desktop apps, web apps, web services, WPF, WCF, Web API etc.
We can discuss any questions and concerns in PM.
Thanks