The subject of the extract is?

Read the extract and answer the question

My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began,
So is it now that I am a man,
So be it when I shall grow
The child is father of the man

A. childishness
B. the adverse effects of the rainbow on man
C. the unchanging cycle of nature
D. the adverse effects of old age

Correct Answer:

Option C – the unchanging cycle of nature