(a) That's wrong. What is important in this example is that the contract involves A in a continuing obligation, to pay a sum to his uncle every year while the horse is alive. When a minor enters into a contract that gives them an interest in property of a permanent nature, or which involves a continuing obligation, the contract can be avoided if the minor chooses to, either before reaching the age of 18, or within a reasonable time afterwards. If the minor does not decide to avoid the contract within this time, they are considered to have decided to continue with it, after which it becomes permanently enforceable. As long as A acts immediately, or within a reasonable time of turning 18, he will be able to avoid the agreement with his uncle.
It should be noted that there is legislation in some Australian states that may affect the extent to which minors may or may not be bound by various types of agreement. This legislation is not uniform and will need to be consulted in specific cases.