The question asks us to identify the specific scientific field that deals with the production, control, and application of very low temperatures using a single word from the given options.

Understanding the Science of Low Temperatures

Let's break down the key phrase: "The science of dealing with the production control and the application of very low temperatures". This describes a specialized area of physics and engineering focused on extremely cold conditions.

Now let's look at the options provided and see which one matches this description:

• Cytology: This is the study of cells, including their structure, function, and life cycle. This field is related to biology, not the study of very low temperatures.

• Chromatics: This refers to the study of color, dealing with aspects like hue, saturation, and brightness. This field is related to optics and physics, but not specifically low temperatures.

• Cryogenics: This term comes from the Greek word 'kryos' meaning 'cold' or 'icy cold'. Cryogenics is indeed defined as the branch of physics and engineering that involves the production and effects of very low temperatures. This aligns perfectly with the description given in the question.

• Cypher: A cypher (or cipher) is a secret way of writing, especially one in which a code is used. This is related to cryptography and communication, not the study of temperatures.

Identifying the Correct Term

Based on the definitions, the science specifically focused on the production, control, and application of very low temperatures is known as Cryogenics. It involves creating temperatures below −150 °C (−238 °F or 123 K) and studying materials and their properties at these extreme conditions.

Therefore, 'Cryogenics' is the most appropriate 'one word' for the given expression.