Good day
I want to connect a MCU to Modbus/TCP network. can i use W5100S? Which chip is the best for this?

Why not? You just need Modbus driver over TCP.

thanks for reply.
My project will work in industry environment, W5100S or W5500, Which one is better for this case?

Comparison matrix. If you will really use the device for industrial applications (which have more strict requirements than consumer/hobbyist), then you should look for the specialists who will be responsible for proper hardware and software design, as well as compliance to requirements.

I personaly would opt for the W5500 (~ 2014) with 8 sockets and a smart hardware ring buffer engine making it very code effective and high performing.
The new W5100S (2018) is a 1:1 re-build of the old W5100 (~ 2007) and limmited to 4 sockets.
Very nice, but I stay with the W5500.
All chips by WIZnet are very good for industrial applications in temperature (-40°C - +85°C) and stability.
A lot of industrial customers use all this chips successfully.

B) Does anyone can recommend a small and good “bare metal” Modbus Server implementation I can use for a ARM Cortex M0 or M3 + W5500 ?

best regards, Joachim