Explanation:
When determining whether an agreement is void or not, one of the factors to consider is whether it is opposed to public policy. Public policy refers to the principles and values that the government and society consider to be in the best interest of the general public. If an agreement goes against these principles and values, it is considered to be against public policy and therefore void.

Examples of agreements opposed to public policy:
1. Trading with an enemy: Engaging in trade with an enemy country during a time of war is considered to be against public policy. This is because it can undermine national security and support the enemy's interests.
2. Trafficking in public offices: Making agreements to buy or sell public offices is against public policy as it promotes corruption and undermines the integrity of the government.
3. Marriage brokerage contracts: Contracts that involve arranging marriages for financial gain are against public policy. They can exploit individuals and compromise the sanctity of marriage.

Contracts to do impossible acts:
Contracts to do impossible acts are not considered to be against public policy. Instead, they are void for being impossible to perform. An agreement can only be enforced if it is capable of being performed by the parties involved. If it involves an act that is impossible to accomplish, it is considered void.

Example:
Suppose two parties enter into an agreement where one party promises to bring a dead person back to life. This agreement would be void as it involves an act that is impossible to perform. However, it is not void because it is opposed to public policy, but rather because it is impossible to fulfill.

Therefore, option D - contracts to do impossible acts - does not cover the idea of an agreement being void due to it being opposed to public policy. The other options, such as trading with an enemy, trafficking in public offices, and marriage brokerage contracts, all involve agreements that are against public policy and would be considered void.