Nature is ___________ and unchangeable, and it is ___________ as to whether its hidden reasons and ___________ are __________to man or not.

The option that best fills the blanks in the above sentence would be:

As per the context, 'relentless', 'indifferent', 'actions' and 'understandable' are the appropriate words.
Hence, option A is the correct answer.

In the second blank, indifferent is the correct choice. as nature is not concert/ unbiased which is the basic idea behind the sentence. Thus Option A or Option D might be correct.

Here if we consider the vastness, unceasingly intense quality is nature then relentless is a better word over harsh.

Option A is correct.