Answer:
The suitable preposition to fill in the blank is "against."
Step-by-step explanation:
The complete sentence is "I will never go against my conscience in taking a decision." The preposition "against" is used to indicate that the speaker will not act in opposition to their conscience. The phrase "go against" means to act in opposition or contradiction to something. In this case, the speaker is saying that they will not act in opposition to their conscience when making a decision. This implies that the speaker values their conscience and believes that it should guide their decisions. By using the preposition "against," the speaker is emphasizing that their conscience is an important guiding principle in their decision-making process.