Question

a would look like the picture below.
concentration gradient
phospholipid bilayer
membrane receptor
molecule diagram

Explanation:

The image shows a distribution of points which could represent the spread of substances from an area of high to low concentration, characteristic of a concentration gradient. A phospholipid bilayer would show a more structured arrangement of molecules in two layers, a membrane - receptor would be more focused on specific binding sites, and a molecule diagram would show the structure of individual molecules rather than a gradient - like distribution.

Answer:

concentration gradient