Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms like humans?
a. Quick diffusion cannot occur in so much cells at a time
b. All cells are not in contact of the atmosphere
c. Air containing intercellular spaces are absent
d. All of the above

Hint: Diffusion is a physical process that refers to the movement of molecules from a high concentration area to at least one of lower concentration. Its rate depends on various factors.

Complete answer:
In order to answer this question we need to know about diffusion and factors that affect it.
one amongst the most characteristics of diffusion is the movement of molecules along the concentration gradient. While this might be facilitated by other molecules, it doesn't directly involve high-energy molecules like nucleotide (ATP) or guanosine triphosphate (GTP). The rate of diffusion depends on the character of interaction between the medium and material.
A gas diffuses very quickly in another gas. An example of this is often the way the noxious smell of ammonia gas spreads in air. Similarly, if a canister of Nitrogen leaks a bit, nitrogen gas that escapes would quickly diffuse into the atmosphere. The identical gas would diffuse slightly more slowly in a very liquid like water and slowest during a solid.
Factors Affecting Diffusion
There are some factors that affect the method of diffusion, which individually and collectively alters the speed and extent of diffusion. These include:
> Temperature.
> Area of Interaction.
> Size of the Particle.
> The steepness of the concentration gradient.

Examples of Diffusion
Diffusion is a vital part of many biological and chemical processes. In biological systems, diffusion occurs at every moment, across membranes in every cell in addition as through the body. For example, oxygen is at the next concentration inside arteries and arterioles, when put next with the oxygen levels in actively respiring cells. By the time blood flows into capillaries within the muscle or liver, as an example, there's only one layer of cells separating this oxygen from hepatocytes or muscle fibers. Through a process of passive diffusion, without the active involvement of the other molecule oxygen passes through the capillary membrane and enters cells. Cells utilize oxygen within the mitochondria for aerobic respiration, which generates \[C{O_2}\] gas as a byproduct. Once again, because the concentration of this gas increases within the cell, it diffuses outwards towards capillaries where the force of flowing blood removes the surplus gas from the tissue region. This way, the capillaries remain at an occasional \[C{O_2}\] concentration, allowing the constant movement of the molecule aloof from cells.
Human beings are multicellular organisms having a complex structure. These systems comprise of tissues which comprise of voluminous cells which require a continual supply of oxygen to fulfill the functional requirements and since these cells aren't connected with the environment round the organism diffusion is insufficient to satisfy the oxygen requirements of multicellular organisms. Also, diffusion could be a slow process which makes it difficult to fulfill the oxygen requirements of the body.

Therefore, the answer is d, all of the above.

Note:
> Diffusion is passive.
> No energy is transferred for it to require place.
> Substances diffuse from a part of high concentration to a part of low concentration. This can be called concentration gradient.
> Rate of diffusion is plagued by temperature because it affects the way that particles move.
> Rate of diffusion is laid low with the area of membrane available. Diffusion is facilitated by membrane proteins.
> The diffusion of water is named osmosis.