Step 1: Understanding the sentence.
The sentence "The panel of scientists was fascinated by the rare species of tree frog, which was capable of producing toxins to even kill a human." is grammatically correct as it stands. The relative clause "which was capable of producing toxins to even kill a human" is properly constructed. The use of "to even kill" is acceptable, and "which was" correctly refers to the tree frog species.
Step 2: Analyzing the options.
(A) was: This is correct. The verb "was" is used appropriately here.
(B) the rare species: This is correct. The phrase "the rare species" is properly used.
(C) which has: This is correct in context, as "which was" is also grammatically acceptable for referring to the species.
(D) to even kill: This part is grammatically correct. The phrase "to even kill" is understandable and accurate.
(E) No Error: There is no grammatical mistake in the sentence. The sentence is correct as written.
Step 3: Conclusion.
The correct answer is (E) No Error, as there is no error in the sentence structure or word usage.