"They are prokaryotes" - YES, both bacteria and Archaea are considered prokaryotes
"They reproduce by binary fission" - YES, but keep in mind (in case your teacher drops this on you in a quiz or exam) that Archaea do not form spores, which many bacteria are capable of
"They are believed to be the ancestors of plants" - ?? Not sure. If I had to guess, I would go with NO, since the general consensus is that plants developed from green algae. Check your class notes.
"They have a cell membrane made of unusual lipids" - NO, only Archaea have that (they possess unusual ether lipids as part of their cell membranes, which bacteria do not)
Explanation: Abrasion means scraping or wearing away. Therefore the correct sample is probably going to look smaller, smoother, and more even from all the scraping of the river currents.
