Which of these espresso variant contain more caffeine? Can you please explain what is the difference between these types of coffee
and which one of them contains more caffeine then the others:


American
Espresso 
Lungo
Ristretto


And please if it's possible to tell the percentage of caffeine of each one.
To say which of these has the most caffeine is a little bit complex, as it depends on if you mean how much caffeine per ounce vs. how much caffeine in the drink in its entirety. To start, let's explain the drinks.

1. Espresso

Espresso is the key to this whole discussion. Each of the four beverages you referenced is (or contains) Espresso. Espresso is a method of brewing coffee, by which pressurized water is passed through a compacted "puck" of fine grounds. In a traditional espresso shot, roughly 0.25 oz of grounds are used to create about 0.85 oz of Espresso. Caffeine levels vary depending on the coffee beans, but as a mainstream reference, Espresso made by Starbucks contains approximately 75mg (they use slightly different measurements, however).

2. Ristretto

Ristretto is a form of Espresso in which less water is used. So you take the same puck of 0.25 oz of finely ground coffee, but instead of making 0.9 oz of Espresso, you make a more concentrated 0.5 oz (or so). Because the strongest Espresso is the first to be brewed, this is the most concentrated form of espresso, and as such has the most caffeine per ounce of any drink on this list. That said, one shot of Ristretto is is less caffeinated than one shot of Espresso simply because there's less of it. As such, if measuring by the amount of caffeine per shot, this is the least caffeinated drink on this list.

3. Lungo

Lungo is the exact opposite of Ristretto. Instead of being an Espresso made with less water, it's an Espresso made with twice the water. As such, a single shot of Lungo has the most overall caffeine of any drink on this list, but is less concentrated than either standard Espresso or Ristretto.

4. Americano

This is Espresso with water in it. No, really. A solo Americano has the exact same amount of caffeine as a solo Espresso, however it is watered down, and as such is the least concentrated drink on this list. You could hypothetically make an Americano from Ristretto or Lungo shots instead of standard Espresso, and in doing so make it less or more caffeinated (respectively).

In Summary

Assuming all are made with the same number of shots, the most caffeinated drinks overall are:


Lungo
Espresso / Americano (tied)
Ristretto


The most caffeinated drinks per oz are:


Ristretto
Espresso
Lungo
Americano