What is the best definition for the underlined word based on the following sentence: "many people can be detrimentally affected by the use of pesticides on crops to prevent them from bug infestations

You did not underline any word, but I found out that the word you are suppose to underline is 'DETRIMENTALLY'
A thing is said to be detrimental if it is capable of causing harm. So, looking at the sentence given in the question, detrimentally in the context of that sentence will mean NEGATIVELY.