I believe you mean Occam's Razor, which means that the simplest answer is usually the right one.
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To get a word meaning "hurry" by adding one letter to "hate," you would add the letter "h" to get the word "haste." The word "haste" means to do something quickly or with urgency. The addition of the letter "h" transforms "hate" into "haste," creating a new word with a different meaning.
Improvise
Acrimonious?
"Adolescent" is one word, and it means "in the process of developing from a child into an adult."
The word sought is "solitary."