Answer:
d. not liable because the consideration is in the past.
Step-by-step explanation:
In order for a valid contract to exist (and then be breached), consideration must be exchanged between the parties involved. E.g. if I just decide to give money to my school because of the good education I got there, it is considered a gift. The consideration from the school took place in the past, it is not exchanging anything with me right now.
The same happens here, the consideration which is the years served is part of the past, nothing is being exchanged in the present. This should be considered a gift from the employer.