Date B.
From SA108 Notes, page 12:
--quote If the disposal (or acquisition) is by way of a contract, the date of disposal (or acquisition) is usually the date of the contract.
In a small number of cases the contract may be conditional. This will be so if one or more conditions have to be met before the contract becomes binding. In these cases the date of disposal (or acquisition) is the date on which the last of the conditions is met.
--unquote
But, surprisingly, it continues:
--quote There are exceptions. For example, in most of the cases where you get a capital sum from your ownership of an asset the date of disposal is the date you receive the capital sum.
--unquote
This almost suggests it could be Date C.