In a construction contract, an employer has agreed to pay a particular price for the services rendered by the contractor. This obligation to pay the agreed price, subject to fulfillment of reciprocal obligations by the other part, is a: $($complete the sentence$)$

a$.$Legally binding obligation, whether reasonable or otherwise.

b$.$Legally binding obligation, only if it is reasonable.

c$.$Legally not a binding obligation, whether reasonable or otherwise.

d$.$Legally, it is not a binding obligation, only if it is unreasonable.