Comparative adjectives are used to compare differences between the two objects they modify (larger, smaller, faster, higher). They are used in sentences where two nouns are compared and 'than' generally used in such sentences.
The sentence in question is a comparison between 'managing present affairs' and 'regretting past mistakes'. Hence the correct comparative adjective to use is option A, 'better'.
Option B is incorrect because 'good' is not a comparative adjective but the base form of 'better'.
Option C is incorrect because 'best' not a comparative adjective but a superlative adjective. Superlatives are not used in comparative sentences; they are used to talk about one thing only and how it is the best or worst etc.
Option D is incorrect as it is an incorrect phrase by itself. This is because 'more' is the comparative of 'much' and 'better' is the comparative of 'good'. It is a rule that we do not use two comparatives together.