To answer this question, we need to understand the concept of an atomic field.
An atomic field is a field that contains a single value. It can only store a single piece of data. This means that it cannot contain multiple values or be used to store collections of data.
Now, let's go through each option to understand why it is correct or incorrect:
Option A) A key that contains only a single field - This option is incorrect because an atomic field is not necessarily a key. It can be any field that stores a single value.
Option B) A key field - This option is incorrect because an atomic field is not necessarily a key. It can be any field that stores a single value.
Option C) A numerical field - This option is incorrect because an atomic field can store any type of data, not just numerical values.
Option D) A single valued field - This option is correct because an atomic field is a field that contains only a single value.
Therefore, the correct answer is D) a single valued field. This option is correct because it accurately describes the concept of an atomic field.