Correct Answer: Probability

Solution : The fourth option is the correct choice.

The most appropriate synonym for " feasibility " is " probability ", because both terms are associated with the likelihood or chance of something happening. Feasibility refers to the practicality or possibility of a proposed plan, while probability refers to the likelihood of an event occurring.

The meanings of the other options are as follows:

• Sustainability: The capacity to endure or maintain practices over the long term.
• Impropriety: Inappropriateness or socially or morally unacceptable behaviour.
• Heavy: Having great weight or density.

Therefore, the correct answer is probability.