Analyzing the Paragraph:
Option 1: This introduces Kervran's initial observation about the hens. Placing the sentence here would prematurely discuss the eggs and their calcareous shells before the hens' behavior (pecking mica) is mentioned, disrupting the logical flow.
Option 2: The paragraph discusses how hens pecked at mica and connects it to the mystery of mica disappearing from their gizzards. Placing the sentence here would shift the focus from the hens' behavior to eggs, which is not yet relevant.
Option 3: The paragraph discusses the unsolved mystery of mica and the hens’ behavior. Adding the sentence here introduces the mystery of eggs with calcareous shells, which is connected to the prior observation but not yet resolved. This placement builds a logical bridge to Kervran's later investigations.
Option 4: After discussing the unexplained observations (hens and their behavior), this is the appropriate place to introduce the specific observation about eggs with calcareous shells. It naturally leads into Kervran's conclusion about element transmutation in the following sentence.
So, the correct option is (C): Option 4.