What Is the Method or Format of Cronbach's Alpha Coefficient

What is the method or format of Cronbach's alpha coefficient as a test for reliability?

A) Comparison of each item in the scale simultaneously with each other item
B) Determination of the correlation between the odd-numbered and even-numbered items of an instrument
C) Determination of a correlation based on the consistency of responses to all items of a single-test form
D) Demonstration of a high degree of agreement when the behaviour or change is observed by two or more observers