The multi-link mechanism in the variable compression turbo engine thus varying the compression ratio.

a. adjusts how far down the piston travels
b. can change one individual piston's operation independent of the other three
c. acts like a fixed-length connecting rod

A. Adjusts how far down the piston travels

Explanation:

This type of engine changes the possition of the piston in order to modify the compression chamber volume and therefore the compression ratio of the engine. The volume of the chamber is proportional to the run of the piston (how far down the piston travels)

This engine is used to achive the optimal compression rate in each individual stage.