Correct option is C. The
The article 'the' is used when the noun that follows is a specific member of a group or individual, either singular or plural in number. The article 'an' is used when the noun that follows begins with a vowel, and it's mentioned for the first time. There are exceptions to this rule: when a superlative adjective precedes the noun (largest bird). In this case, the article 'the' is used, because there can be only one largest species of birds. The article 'a' would be an incorrect option because it does not refer to any ostrich in general, but the entire species. Plus if it was about any ostrich bird, 'an' would be used. An article is required in front of 'ostrich' in the sentence, since it's neither a proper noun, nor a non-count noun, so option D is incorrect.