Step 1: Understand the cell membrane.
The cell membrane is the thin, living, outer boundary of a cell. It controls what enters and leaves the cell.
Step 2: Name the model.
The most accepted picture of the membrane is the Fluid Mosaic Model, given by Singer and Nicolson in 1972.
Step 3: Understand the word fluid.
The phospholipid molecules of the membrane are not fixed. They can slide sideways, which gives the membrane a fluid, flowing nature.
Step 4: Understand the word mosaic.
Many proteins sit within the phospholipid layer. Scattered about, they look like the small pieces of a mosaic design.
Step 5: Rule out the other names.
Schleiden and Schwann gave the cell theory, Watson and Crick described the structure of DNA, and Robert Brown discovered the nucleus. None of them gave this model.
Step 6: Conclude.
So the Fluid Mosaic Model was proposed by Singer and Nicolson.
\[ \boxed{\text{Singer and Nicolson}} \]