Questions: All microorganisms are best defined as organisms that - lack a cell nucleus. - cause human disease. - can only be found growing in laboratories. - are too small to be seen with the unaided eye. - are infectious particles.

The answer is the fourth one (D): are too small to be seen with the unaided eye.

Explanation for each option:

1. Lack a cell nucleus: This is incorrect because not all microorganisms lack a cell nucleus. For example, bacteria lack a nucleus, but fungi and protozoa, which are also microorganisms, have a nucleus.

2. Cause human disease: This is incorrect because not all microorganisms cause disease. Many microorganisms are harmless or even beneficial to humans, such as those involved in digestion or those used in food production.

3. Can only be found growing in laboratories: This is incorrect because microorganisms are found everywhere in nature, not just in laboratories. They exist in soil, water, air, and within other organisms.

4. Are too small to be seen with the unaided eye: This is correct because microorganisms, by definition, are organisms that are microscopic and cannot be seen without the aid of a microscope.

5. Are infectious particles: This is incorrect because not all microorganisms are infectious. While some microorganisms, like viruses and certain bacteria, can be infectious, many others are not.

In summary, microorganisms are best defined as organisms that are too small to be seen with the unaided eye.