The most important product of the electron
transport chain in cellular respiration is

ATP

Oxygen is the final electron acceptor in the electron transport chain, showing the need for aerobic conditions to undergo such a process. ATP is produced as a product of the electron transport chain, while glucose and CO2 play a role in earlier processes of cellular respiration.

option c is the correct answer.

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