Which wavelength of light color shows the maximum absorption capacity by chlorophyll '$a$'?

The molecular formula of chlorophyll $a$ is:

Which chlorophyll molecule does not possess a phytol tail?

Grana refers to

In the figure given below,the black line (upper) indicates the action spectrum for photosynthesis and the lighter line (lower) indicates the absorption spectrum of chlorophyll-$a$. Answer the following:
$(a)$ What does the action spectrum indicate? How can we plot an action spectrum? Explain with an example.
$(b)$ How can we derive an absorption spectrum for any substance?
$(c)$ If chlorophyll-$a$ is responsible for the light reaction of photosynthesis,why do the action spectrum and absorption spectrum not overlap?

$A$ flattened disc-like sac in a chloroplast is called a