Why B? And why not the others?

Correct answer: B. That different knowledgeable and independent observers could reach consensus, though not necessarily complete agreement, that a particular depiction is a faithful representation

Chapter 3 of Concepts Statement No. 8 defines verifiability as meaning that different knowledgeable and independent observers could reach consensus, although not necessarily complete agreement, that a particular depiction is a faithful representation, which helps assure users that the information reasonably reflects economic reality rather than one preparer's unchecked assertion. The option describing traceability to a single source document is wrong because verifiability is about independent observers being able to corroborate a depiction, not merely about a paper trail to one document, and some verifiable estimates have no single underlying source document at all. The option describing consistent use of the same methods period after period is wrong because that describes comparability, a separate enhancing characteristic concerned with the ability to identify similarities and differences across periods or entities, not with independent corroboration. The option describing early availability to influence decisions is wrong because that describes timeliness, another distinct enhancing characteristic, not verifiability.

Source: FASB Concepts Statement No. 8, Chapter 3, Qualitative Characteristics of Useful Financial Information