John, a building contractor, agreed to pay a penalty if he fails to complete the construction work on certain date. However, before its completion date, John and the owner, Peter, agreed that John should do some additional work on the same project. The changes made it impossible for John to complete the building by the original date. Peter insisted that the penalty clause allowed him to deduct the penalty from the amount he still owed to John. The above case in an example of:

a$.$

Novation.

b$.$

All the above.

c$.$

Waiver

d$.$

Tender of performance