Which of the following is the source of carbon for plants?

The primary mode of nutrition in plants is -

What is the actual concentration of $CO_2$ in the atmosphere (in $\%$)?

Of the incident solar radiation,__$(A)$__ is photosynthetically active radiation $\text{(PAR)}$. The correct option for the above blank '$A$' is:

How is the middle of the $19^{th}$ century an important reference to photosynthesis?

The hydrogen donor in bacterial photosynthesis is usually