This is the 8117th most frequent Japanese word.
対する
"To face," "to confront," "to oppose," or "to be directed toward."
The term 対する signifies 'against' or 'in response to', indicating a directional relationship towards his opinion.
彼の意見に対する反論が必要です。
A rebuttal against his opinion is necessary.
Here, 対する indicates 'regarding' or 'towards', showing an involvement with the proposal presented.
その提案に対する否定的な意見を表明しました。
I expressed a negative opinion regarding that proposal.
In this instance, 対する means 'towards', expressing an orientation of interest concerning environmental issues.