Final answer:
Rhinoceros horn is the by-product that is marketed as an aphrodisiac in the Far East, which has led to grave international conservation issues.
Step-by-step explanation:
The by-product that is marketed as an aphrodisiac in the Far East is B) Rhinoceros horn. Despite its popularity in certain cultures, there is no scientific evidence supporting the effectiveness of rhinoceros horn as an aphrodisiac. Unfortunately, the demand for this by-product has led to severe negative effects, including the threat of extinction for rhinoceros species due to poaching. This issue raises significant environmental, conservation, and ethical concerns internationally.