Final answer:
The predefined field size that cannot be changed in Microsoft Access is associated with the 'Data Type' of a field. Once a data type is assigned to a field, its format typically cannot be altered without modifying the table's structure.
Step-by-step explanation:
The correct answer to the question is 3) Data Type. In Microsoft Access, when you define a field in a table, you assign a data type to that field which determines the kind of data it can hold. For example, a Text data type field can store alphanumeric characters, whilst a Number data type can only hold numeric values. Once the data type is set for a field, it has a predefined format that usually cannot be changed without creating a new field or altering the structure of the table considerably.
It's worth noting that a Primary Key is a field (or set of fields) whose value uniquely identifies each record in a table. A Foreign Key is a field in one table that uniquely identifies a row of another table, thus, it is used to define a relationship between two tables. An Index is used to speed up the search and sorting operations within the database.