In the given sentence, we are talking about one giant tortoise. When the noun is singular an article must be used before it.
'A' is used in front of words that start with a consonant sound.
'An' is used in front of words that start with a vowel sound.
'The' can be used with both singular and plural nouns, however the usage of 'the' denotes that the denoting person or thing is already mentioned or known by the reader or listener.
Thus, in the given sentence the apt answer is Option B, 'a'.