Final answer:
The phrase 'its foundations' most appropriately completes the sentence, indicating the start of an unending decline after a house's structural integrity is compromised.
Step-by-step explanation:
The question "For when a house hath once been shaken from _____________, there the curse fails nevermore." pertains to a line that suggests the beginning of an irreversible decline or curse after a house's stability has been compromised. The phrase that best completes this sentence from the options given is likely 'its foundations', as the foundation is typically what supports the entire structure of a house and once it has been shaken, the stability of the whole house is in jeopardy. This metaphor can be used for various situations where an initial serious damage leads to continuous problems.