Question

a challenge to traditional (pre - 1860) ideas about species came from embryology, when it was discovered that
all animals develop similar embryonic gills
mutations have a far more dramatic effect on embryos than on adult organisms
the more advanced the animal, the more slowly it develops
embryos of dissimilar organisms, such as sharks and humans, show more similarities than appear in adult forms
the embryological development of many plants and animals is almost identical

Explanation:

Embryology showed that embryos of different organisms like sharks and humans have more similarities than their adult - forms, challenging traditional species ideas. Other options are incorrect: not all animals have similar embryonic gills; while mutations can affect embryos, this wasn't the key challenge here; there's no clear relation between animal advancement and development speed; and plant and animal embryological development is not almost identical.

Answer:

embryos of dissimilar organisms, such as sharks and humans, show more similarities than appear in adult forms