Hey there! Thanks for answering so quickly! 
The window in the screenshot you posted is the one that appears when I click Plank’s icon in the dock… And it used to show the “Icon Zoom” option like the one in your picture… But now that’s gone… Even if I launch the preferences window from the Terminal…
I wouldn’t know what version of Plank I’m using, because it doesn’t say it anywhere…
All I know is that the Software Updater offered me to get a new version today, and I did update it…
My window manager is Marco With Software Compositor, I had never changed that, but when I tried it a couple of minutes ago, all the other ones seemed to have glitches, so I reverted back to that one…