Explanation:

Correct Answer is (e).When we talk about the development of the mental processes being most apparent, we need an adjective that will describe this progress. Options A and B, thus, can be eliminated. 
“Stimulus” means ‘an agent or action or condition that elicits a response’ and usage of this for the first blank is also irrelevant. Option D gets eliminated as well.
We could consider option 3, as ‘nature of development’ sounds logical. However, when we look at the option for the second blank- “celestial” which means ‘relating to the heavens’ or ‘divine’, this does not sound correct. So option C can be eliminated too.
“Chronological” means ‘arranged in order of time of occurrence’ and logically fits the second blank.
Option E is hence the correct answer.