Which of the following components of the electron transport chain passes electrons to O2 and causing it to be reduced to H20? A. Cytochrome c oxidase
B. Succinate -Q reductase
C. NADH
D. NADH-Q oxidoreductase

The correct answer is option A, that is, cytochrome c oxidase.

Explanation:

A sequence of complexes, which conduct electrons from electron donors to electron acceptors through redox reactions, that is, both oxidation and reduction reactions taking place simultaneously is termed as the electron transport chain. This conduction of electrons is further combined with the mediation of protons across a membrane.  

Four membrane-bound complexes are present within the mitochondria, of these in complex IV or cytochrome c oxidase, the withdrawal of 4 electrons takes place from 4 molecules of cytochrome c, and is mediated towards the molecular oxygen, which further gives rise to 2 molecules of water. During the process, 8 H+ are withdrawn from the mitochondrial matrix, which contributes to the proton gradient.  

it's diluted or concentrated

in a diluted solution, you can add a lot more solute but in concentrated only a little more can be added