There ... a big elephant in the Zoo.

The sentence is singular, referring to 'a big elephant'. Therefore, the singular form of the verb 'to be' in the past tense is required. The correct option is 'was'. However, 'was' is not an option. Among the given options, 'is' is the correct singular form for the present tense. Since the sentence structure implies a present state, 'is' is the most appropriate choice. The sentence should be 'There is a big elephant in the Zoo.'

Answer: is