In lipid bilayer, lipids are arranged with the polar head towards the outer side and the hydrophobic/ non-polar tails towards the inner side.

RBCs lack a nucleus and may other cell organelles thus they are best suited to study cell membrane without any complication.

In $1935$, Danielli and Davson proposed that cell membrane is made of a double layer of phospholipid molecules sandwiched between two single layers of proteins. The three layers are held together by electrostatic forces while phospholipid layers are kept adhered by vander Waal's forces.

• The fluid mosaic model of the biomembrane or cell membrane was proposed by SJ Singer and GL Nicolson in the year 1972.
  
• According to this model, the protein molecules are embedded in the phospholipid bilayer.
  
• Thus, the proteins are seen as icebergs in the sea of lipids.
  
• The lipid bilayer of the cell membrane provides fluidity, flexibility and elasticity to the membrane. 
  
• So, the correct answer is 'Protein embedded in phospholipid bilayer'.

• The cell membrane or plasmalemma is a phospholipid bilayer that are selectively permeable membranes which separate the interior of the cells (cell organelles and cytoplasm) from the outside environment or the extracellular space.
  
•  A selectively permeable can be defined as a cell membrane which allows only certain molecules or ions to pass through the membrane by active or passive transport.
• So, the correct answer is 'Cell membrane'.

• Most unicellular organisms are of microscopic size and are thus classified as microorganisms, for example, Phytoplankton and protozoa range from about 0.001 mm to about 0.25 mm.
• Most of the multicellular organisms and vertebrates are visible to the naked eye, which means that it is possible to view them without using the microscope.
  So, option A is the correct answer.
  

• Cytoskeleton refers to the complex network of interlinking filaments and tubules which extend throughout the cytoplasm of the cell from the nucleus to the plasma membrane.
  
• They are composed of microtubules, actin filaments and intermediate filaments. 
• They give shape and support to the cell and also facilitates movement within the cell (cytoplasmic streaming or cyclosis).
• It was discovered by Nikolai K. Koltsov in the year 1903.
  
• So, the correct answer is 'Koltzoff'.

Cytokinesis is the process of division of Cytoplasm. "Kinesis" means division and "cyto" refers to cytoplasm. it occurs during cell division. It is variable from plant cells to animal cells.

Stem cells are totipotent cells that can able to differentiate and divide into any type of cells of the body. meristematic cells are undifferentiated cells of the plants that can able to divide and form tissues, organs of the plant. so cells capable of division are all the cells including stem cells, meristematic cells and undifferentiated cells. Hence option D is correct.

Mammalian erythrocytes also called as red blood cells are formed in bone marrow from multipotential hemocytoblast and further converted to erythroblast (normoblast). The nucleus and mitochondria of erythroblast disappear and it is converted to mature erythrocyte with haemoglobin. So RBC's are differentiated cells which lack a nucleus.

An undividable differentiated cell sometimes gains the power to divide. This process is called dedifferentiation. Wound healing after injury and secondary growth in plants are some of the best examples of dedifferentiation. Dedifferentiation is one of the mechanisms of natural regeneration. So answer D is correct.