Blood

Step 1: Understanding platelet extraction.
Platelets are typically extracted from plasma by a centrifugation process, which allows the separation of different blood components based on their density. Platelets form a pellet at the bottom, separated from the plasma.
Step 2: Analyzing the options.
(1) Autoclave: Incorrect, autoclaving is used for sterilization, not for separating platelets.
(2) Filtration: Incorrect, filtration is not commonly used for platelet extraction.
(3) Dialysis: Incorrect, dialysis is used to separate molecules based on size, not for platelets.
(4) Centrifuge: Correct — Centrifugation is the method used to extract platelets from plasma.
Step 3: Conclusion.
Platelets are extracted using (4) Centrifuge.