Headings:
- Introduction
- Meaning of public policy
- Examples of agreements opposed to public policy
- Explanation of each option
- Conclusion

Introduction:
An agreement is a legally binding contract between two or more parties. However, if an agreement is opposed to public policy, it is considered void and unenforceable. In this question, we have to identify which option is not covered by heads of public policy.

Meaning of public policy:
Public policy refers to the principles and guidelines that govern the actions of the government and its citizens. It is a set of rules and regulations that are established to promote the welfare of the public. In the context of contract law, public policy is used to determine whether an agreement is legal and enforceable.

Examples of agreements opposed to public policy:
- Agreements that are immoral or illegal
- Agreements that are against the public interest
- Agreements that are oppressive or unconscionable

Explanation of each option:
a) Trading with an enemy: Trading with an enemy is considered against public policy as it goes against the interest of the nation. Any agreement that involves trading with an enemy is void and unenforceable.

b) Trafficking in public offices: Trafficking in public offices means buying or selling government positions for personal gain. This is considered against public policy as it undermines the principles of democracy and good governance. Any agreement that involves trafficking in public offices is void and unenforceable.

c) Marriage brokerage contracts: Marriage brokerage contracts refer to agreements between two parties for arranging a marriage. While these contracts are not illegal, they are considered against public policy as they commodify human relationships. However, if such contracts are entered into in good faith and with the intention of facilitating a marriage, they may be enforceable.

d) Contracts to do impossible acts: This option is not covered by heads of public policy as it does not involve any immoral, illegal, or oppressive conduct. Contracts to do impossible acts are void as they are impossible to perform, but they are not considered against public policy.

Conclusion:
In conclusion, contracts that are opposed to public policy are void and unenforceable. Trading with an enemy and trafficking in public offices are examples of agreements that are against public policy, while marriage brokerage contracts may or may not be enforceable depending on the circumstances. Contracts to do impossible acts are not considered against public policy.