Smaller, lipid-soluble molecules diffuse faster through the cell membrane, but the movement of hydrophilic substances is facilitated by certain transporters which are chemically
(a) Proteins
(b) Carbohydrates
(c) Lipids
(d) Phospholipids

Hint: These are macromolecules formed by amino acids. They are large size molecules (macromolecules), polymers of structural units called amino acids. These are the building blocks of the body that provide energy of 4 cal/gram.

Complete Step by Step answer:
Particles which are lipid-soluble (i.e., lipophilic or hydrophilic) can easily diffuse through the cell membrane as the lipid is a major constituent of the membrane. Whereas hydrophilic substances (i.e., the substances which are soluble in water, e.g., glucose) do not pass through a biological membrane by simple diffusion. They are, therefore, transported across a cell membrane by facilitated diffusion (i.e., carrier- mediated diffusion), which takes place with the help of certain transport proteins.
So, the correct answer is 'Proteins'.

Additional Information:
Active transport moves molecules and ions from lower concentration to higher concentration with the assistance of energy in the form of ATP. On the opposite hand, passive transport moves molecules and ions from a higher concentration to lower concentration without the use of energy.
There are two major ways in which molecules are often moved across a membrane, and therefore the distinction has to do with whether or not cell energy is used. Passive mechanisms like diffusion use no energy, while active transport requires energy to make it done.

Note:
- Carrier proteins are proteins that help in the movement of ions, small molecules, or macromolecules, like another protein, across a biological membrane. Carrier proteins are integral membrane proteins; that's, they exist within and span the membrane across which they transport substances.
- Transport proteins function in both active and passive transport to transport molecules across the plasma membrane. Two main groups of transport proteins are often found within the plasma membrane, and each helps water-loving molecules pass through the plasma membrane.