How slave data send to master?

I have system like one master and 20 energy meter as slave device.
I am reading fixed 10 holding register and 10 coil register from each slave.
Baud rate : 9600,
Parity : even,
Data bit : 8
Response timeout : 3 second.

My questions are :

How slave will receive data from slaves?
1) First holding register from first slave and then second slave's first holding register and then third.... up to 2oth, and again second holding register from first slave and second register from second slave like this...

2) first of all, all holding register of first slaves and all coil register of first slave and then second slave's turn. like this goes to last 20th coil register and start again first slave?

How is it work?

One of this way is it working or third option is there????