The correct option is D curtail
The word 'prolong' refers to extending the duration of.
Option A, obstruct refers to being or getting in the way.
Option B, curtail refers to reducing in extent or quantity.
Option C, hinder refers to making it difficult for someone to do something for something to happen.
Option D, restrain refers to preventing someone from doing something.
Thus, according to the meaning, the word 'curtail' is the opposite of the word 'prolong'.