What is common law illegality? Which of the following is correct?

a)Common law determines that some agreements may be void, but this does not necessarily make them unenforceable as they would be under statute

b)If a contract is illegal under common law it is possible to apply to a court who can validate the contract, thereby overcoming the impediments set out in common law

c)Where the common law determines that a contract is against public interest in some manner then the agreement is void or at least unenforceable

d)Common law illegality only applies to taxation offences, tax avoidance agreements are voided, even though not covered by legislation