Assumption. The correct answer choice is (B)
Because the troop of macaque monkeys adopted the behavior of spending time in the hot spring, the author concludes that the monkeys are able to adopt and pass on new patterns of social behavior. Notice that the argument explicitly rejects genetics as the cause for this behavior, stating that these macaques "are not complete captives of their genetic heritage":
- Cause Effect
Adopt/pass-on → Jump in hot spring
Whenever the author accepts a particular cause over another, her argument depends on assuming that the rejected cause played no role in constituting the accepted one. What if the ability to adopt and pass on new patterns of social behavior were itself a product of genetic heritage? If true, the argument would be illogical: therefore, the author assumes that such ability was not genetically predetermined. Answer choice (B) is correct.
Answer choice (A): If mutations in the genetic heritage can occur over a time span as short as a few years or decades, then perhaps the monkeys' behavior was the product of genetic mutations. This answer choice does the exact opposite of what is needed — it supports the alternative cause for the stated effect and is therefore incorrect.
Answer choice (B): This is the correct answer choice. See discussion above.
Answer choice (C): Since the behavior in question did become typical among the macaque monkeys, this answer choice gives us no reason to reject genetic alterations as the probable cause for such behavior. This answer choice does the exact opposite of what is needed — it supports the alternative cause for the stated effect and is therefore incorrect.
Answer choice (D): This is perhaps the most attractive decoy answer. If the social behaviors of monkeys were completely independent of their genetic heritage, the conclusion is justified as true. However, notice that this is not a Justify the Conclusion question: we are not asked to strengthen the argument 100%. Our job is merely to state a necessary, missing premise for that conclusion. The argument does not rely on the idea that all social behaviors are completely independent of genetics. Rather, we only need to show that some social behaviors are not entirely dependent on genetics. This answer choice uses language that is too strong for Assumption questions. Always ensure that you answer Assumption questions with the most "minimalist" answer possible — the one best tailored to the particular conclusion at hand.
Answer choice (E): It is irrelevant whether this pattern of behavior will persist over several generations or not, as this can be due to either genetics or environmental factors. This answer choice is incorrect.