Choose the correct alternative which will improve the part of the sentence given in quotes.
Some children are much more 'aggressive then' others.

Correct option is B

The correct substitution of the quoted words is "aggressive than".
Explanation: The comparison between the levels of aggression among children should use the comparative "than" rather than "then" which is a temporal conjunction. The adjective "aggressive" should remain unchanged as it correctly describes the quality being compared.
Grammatical rule used: Comparatives are typically followed by "than" when making comparisons.
Example: He is more intelligent than his brother.