Answer:
a. is actually variable.
Step-by-step explanation:
Earth's solar constant -
It refers to the total amount of radiations coming from the sun per unit area and per unit time on the surface of the earth , is referred to as Earth's solar constant .
There is an increase of about 0.2 percent during the peak of every 11 year solar cycle and hence ,
The amount is variable in nature .
Since , the sun is composed of hydrogen gas , which increases to around 10 percent on every billion year .
Hence , from the given question ,
The correct option is a. is actually variable.