Once a contract is signed, it is legally binding on all sides unless?

My opinion is that the right answer is:

c

the most suitable answer is option C:   It violates an applicable law Buyer is unable to finance the work 

C. is te most suitable.............

Ans: C (Violation of T&C of contract agreement)

Yes, a signed contract is legally binding. If either party does not honour the contract, the complainant party can approach the relevant civil law enforcement institution for enforcement of its execution or revoke penalty provisions. Therefore

A: Buyer can revoke penalty provisions.

B: Contract can not be declared Null & Void without mutual consent. Either party lawyer alone can not declare it null and void unless this right has been given to any party in signed contract.

D: It can form be underlying reason as mentioned in option (may be buyer is bankrupt) for cancellation for violation of payment T&Cs.

Thanks for invitation - 

With the choice - C .

b.   Either side's lawyers declares it null and void. 

When contract is signed, it becomes a legal binding for both the parties unless they mutually agree to withdraw from it and declare  the contrcat as null and void 

unless it is agreed on writing  and by all parties that the contract is violated due to its illegality with the local legislation , or causes some hazard to  inhabitants in the vicinty ,or causing enviromentalbad effect or obstructing traffic , or some civil activities or the need to modify the contract it self due to unexpected changes like change of prices or deevauation of currency or even that no bidding or the information of the bid price had leaked from some source , or if the cotract is done with a conflict of inerest to someoutsider

Thanks for invitation....I endorse answers given by colleagues &Experts

Option " d '" appears to be most appropriate in the given situation. Option " c " can also be made the reason for contract cancellation but it involves convincing the vendor.

b.   Either side's lawyers declares it null and void. 

thanks for invition ,,,,,,,,,, my choice is option C