Why B? And why not the others?

Correct answer: B. IAS 41 requires the herd to be measured at fair value less costs to sell, with changes recognized in profit or loss; US GAAP has no equivalent standard, and such assets are generally carried at historical cost

IAS 41 applies to biological assets such as living animals and plants used in agricultural activity, and it requires them to be measured, both on initial recognition and at each subsequent reporting date, at fair value less estimated costs to sell, with the resulting gain or loss recognized in profit or loss for the period in which it arises, except in the narrow case where fair value cannot be reliably measured on initial recognition. US GAAP has no equivalent comprehensive standard applying a fair value model to biological assets generally; livestock and similar agricultural assets held by most entities are typically carried at historical cost, consistent with the broader US GAAP preference for cost-based measurement, with only limited industry-specific guidance addressing certain agricultural producers. The option requiring fair value under both frameworks is wrong because it overstates US GAAP's treatment, which lacks IAS 41's general fair value mandate. The option swapping the frameworks is wrong because it attributes the dedicated fair-value agriculture standard to US GAAP and a cost-only rule to IFRS, which is the reverse of the actual standards. The option prohibiting recognition of living animals as assets under either framework is wrong because both frameworks recognize such animals as assets when the entity controls them and future economic benefits are expected to flow to the entity; the disagreement is only about how they are subsequently measured.

Source: IFRS Foundation, IAS 41 Agriculture, paragraphs 10-13 (recognition and fair value measurement of biological assets); US GAAP has no directly equivalent comprehensive standard, so agricultural assets are generally measured at historical cost