Several important mechanisms regulate regeneration across diverse organisms, each involving different cellular and molecular strategies. The following analysis evaluates statements related to blastema cell behavior, signaling pathways, and regenerative capacities.

Statement Analysis

• A. Amphibian blastema cells retain their specification during dedifferentiation:
  Blastema cells in amphibians dedifferentiate but retain positional memory or specification relevant to their tissue origin. They do not become entirely totipotent but are lineage-restricted, maintaining some differentiation identity important for proper regeneration.

• B. Transgenic Hydra with ubiquitous β-catenin misexpression shows numerous ectopic tentacles:
  Experimental evidence indicates that global β-catenin misexpression in Hydra leads to ectopic buds and axes, not just tentacles. The main phenotype is formation of multiple ectopic buds and heads along the body axis.

• C. Planarian Wnt pathway activation causes the posterior blastema to regenerate a head:
  This is actually incorrect. In planaria, Wnt pathway activation promotes posterior identity; thus, activation in the tail blastema results in tail regeneration, not head. Conversely, inhibition of Wnt signaling in posterior regions can induce ectopic heads.

• D. Regenerating blastema is formed in the mammalian liver:
  Mammalian liver regeneration usually occurs through compensatory hyperplasia of existing mature hepatocytes rather than formation of a blastema. Although there is proliferation, a blastema-like structure like in amphibians does not form.

Conclusion

• Statement A is correct; amphibian blastema cells retain lineage specification.

• Statement B about Hydra ectopic tentacles is inaccurate as the phenotype includes ectopic buds and axes.

• Statement C is incorrect regarding head regeneration upon Wnt activation; Wnt promotes tail identity.

• Statement D is incorrect; mammalian liver regeneration does not involve blastema formation.

Summary

The only correct statement from those given is that amphibian blastema cells retain their specification during regeneration (A only).

Final Answer:
(1) A only