I shared my phone's MAC id to connect to my office wifi. Will the system administrator be able to see my whatsapp messages or internet activities in my phone ?
The Media Access Control address is simply used to allow the wireless router to find your device across the radio frequency, and to identify your phone when you want to speak to the router's radio antenna. It is not used, or visible, outside the wireless network, so is not used as part of any secure connections to services outside that network.
Secure services like WhatsApp are not vulnerable to inspection by simply capturing the traffic, as they use end-to-end encryption. Websites and apps that do not use encryption can be inspected by the network operator.
The unique risk versus an open wireless network is that the network operator can now use that MAC address to correlate network traffic with your device. If they notice unwanted traffic, they can see the MAC address participating in that traffic and know that you are the owner/operator and probably responsible for that traffic.
There is generally a table on the router where IP addresses are matched to mac addresses anyway so any time you are using a network you are probably sharing a mac address. You want the router to route traffic to you and not me, so it has to know which traffic is yours and which is mine.
If you are using encryption (including https) the content of your traffic is not visible. Meta-data still can be very telling though; they might not see what you did at [site_that_gets_you_fired], but they might be able to see that you visited.
The difference between connecting to a totally open network and a mac address required network is you are giving a mac in person, so rather than note a device (or its clone) is connected they have at least a physical description of you to match to that device.