In animal farm why does he describe the animal as slaves in chapter 6

During the following year, the animals work harder than ever before. Building the windmill is a laborious business, andBoxer proves himself a model of physical strength and dedication.Napoleon announces that Animal Farm will begin trading with neighboring farms and hires Mr. Whymper, a solicitor, to act as his agent. Other humans meet in pubs and discuss their theories that the windmill will collapse and that Animal Farm will go bankrupt. Jones gives up his attempts at retaking his farm and moves to another part of the county. The pigs move into the farmhouse and begin sleeping in beds, which Squealerexcuses on the grounds that the pigs need their rest after the daily strain of running the farm.

So I searched for your question and discovered there where more colors to the car! So for the sake of example, I will give my example, and still answer your question! 

Okay, so at the top of the diagram, you start the basics, 2 cars. 

Next, you want to go into detail, the model, Sports, luxury, or standard! 

Then for each model, you have colors! silver, green, blue, and red.
 

To answer your original question, there would be 6 choices/options to choose from.  
 - Because they specifically wanted a red car, and there are 3 models, and 2 car builds.  Multiply 2 and 3, you have 6, or make a diagram. 

To explain and answer my diagram/example, 
there would be 24 choices/options for the family to choose from. 
-Because there are 4 different colors, 3 different model types, and 2 car builds. Multiply 4,3, and 2, and you get 24!  (You can also just count the colors on the diagram!) 

(To answer the model question I found while searching online, the answer would be 30 choices.)