Which gas is used by plants for respiration? 

Oxygen 

The correct answer is option 1. Oxygen.

Plant respiration is a biological process that involves the exchange of gases, specifically oxygen and carbon dioxide. During respiration, plants take in oxygen from the surrounding environment and use it to break down sugars and other organic molecules to release energy for their metabolic activities. The simplified equation for plant respiration is:

\(C_6H_{12}O_{6} + 6O_{2} \longrightarrow 6CO_2 + 6H_2O + Energy\)

In this process, glucose \((C_6H_{12}O_6)\) reacts with oxygen \((O_2)\) to produce carbon dioxide \((CO_2)\), water \((H_2O)\), and energy. The released energy is then used by the plant for various cellular functions and growth.

Contrastingly, during photosynthesis, plants use carbon dioxide and release oxygen. This process occurs in chloroplasts and is the opposite of respiration:

\(6CO_2 + 6H_2O + light energy \longrightarrow C_6H_{12}O_6 + 6O_2\)

In summary, while plants produce oxygen during photosynthesis, they consume oxygen and release carbon dioxide during respiration, utilizing the oxygen for their metabolic needs.