When performance of an obligation has become illegal subsequent to the contract but prior to performance, the contract will most likely be terminated on the basis of .

impossibility

Explanation:

When there is  unforeseen circumstances held and performance of contract is not possible under such circumstances then contract is terminate on basis of impossibility of contact. In contact law, when one party terminates the contact on basis of unforeseen circumstances that not allowed to continue the performance known as impossibility.

answer; they are called (pebbling);