The main difference between Archaebacteria and Eubacteria is?

Hint: Archaebacteria are called ancient bacteria whereas eubacteria are called true bacteria. Unlike eubacteria, archaebacteria can survive in extreme conditions.

Complete answer:
Kingdom Monera is classified into two domains namely archaebacteria and eubacteria. Kingdom Monera contains the least organized unicellular prokaryotic microorganisms on earth. Both archaebacteria and eubacteria are unicellular microorganisms, which are usually called prokaryotes. Archaebacteria are usually found in extreme conditions whereas eubacteria are found everywhere on the surface of Earth.
There are several characteristics of the plasma membrane that are unique to Archaea and set them apart from other domains. One such characteristic is the chirality of the glycerol linkage between the phospholipid head and the side chain. In archaea, it is generally in the L-isomeric form, while bacteria and eukaryotes have the D-isomeric form. A second difference is the presence of an ether-linkage as opposed to the ester-linked lipids found in eubacteria and eukaryotes. The ether-linkage also provides more chemical stability to the membrane in contrast to ester linkage. Unbranched fatty acids can only be found in eukaryotes, while isoprenoid chains are found in archaea.

Additional information:
Some differences between archaebacteria and eubacteria:
-Cell Wall
Archaebacteria: Cell wall is mainly composed of pseudo peptidoglycans.
Eubacteria: The cell wall is mainly composed of peptidoglycans with muramic acid
-Growth and Reproduction
Archaebacteria: Asexual reproduction- binary fission, fragmentation, and budding.
Eubacteria: In addition to other asexual methods they also produce spores in order to remain dormant during unfavorable conditions.
-Types
Archaebacteria: Major types are: methanogens, halophiles, and thermophiles.
Eubacteria are classified into two types: gram-positive and gram-negative.

So, the correct answer is the cell membrane structure.

Notes:
The plasma membrane of Archaea is generally found as monolayers, where the isoprene chains of one phospholipid connect with the isoprene chains of another phospholipid on the opposite side of the membrane. Bacteria and eukaryotes only have lipid bilayers, where the two sides of the membrane remain separated.