The correct option is A tallest
The phrases in a comparison sentence should both be parallel in structure as well as in context. Here there are two superlatives concerning the same noun 'giraffe' in comparison with the other animals. The underlined superlative 'best' attributes the meaning 'desirable quality', which is incorrect in the context of the sentence as it's a quality in terms of personality, and it's not relevant here. Hence option D is incorrect. The phrase 'largest ruminant' suggests that we're describing the noun 'giraffe' in terms of its size, the only relevant option is Option A, which modifies the noun 'terrestrial animal' in terms of height, hence option A is correct. Option B is incorrect because it's a comparative, whereas the noun calls for a superlative. Similarly, option C is incorrect because 'herbivorous' is not a superlative adjective.