Which tooth anomaly is described as failure of normal invagination of Hertwig's epithelial root sheath?

The correct choice is taurodontism, which is characterized by the failure of normal invagination of Hertwig's epithelial root sheath during tooth development. This anomaly results in a tooth that has an elongated pulp chamber and short roots. The alteration occurs due to the disturbance in the normal shaping of the root, leading to a distinct appearance where the pulp chamber is enlarged and the roots may be flattened or less pronounced.

In contrast, the other options describe different dental anomalies. Fusion involves the joining of two adjacent teeth at the developmental stage, leading to a single larger tooth structure, while gemination refers to the incomplete division of a single tooth bud, resulting in a tooth with two crowns and a single root. Concrescence describes the joining of two teeth at their roots by cementum, typically after their formation has been completed. Each of these conditions arises from different developmental processes or disturbances, further clarifying why taurodontism is the correct answer in this context.