"Hij hoort bijna alle opmerkingen."
Translation:He hears almost all the comments.
Sounds like the emphases is wrong on opmerkingen - should be on op instead of merk.
A "bericht" can contain an "opmerking" and the "opmerking could be a "reactie" on something.
E.g. you write a letter or email (these would be the "bericht"). Your letter could contain a complaint or suggestive ideas to a company. Such an idea or complaint is a "opmerking". This can be many things. The whole picture of you sending a letter is the "reactie" on something. E.g. you writing the letter because the thing you ordered was broken on delivery.
Alot of text but I hope this helps a bit. Cheers.