'A' and 'an' are indefinite articles used for nouns which are not specific, indefinite, and singular. The birds mentioned are plural hence options B and C are incorrect. 'Some' is a quantifier which is usually used in a positive statement. 'Some' means an unspecified quantity or number, which is little or less. Hence it's the correct option here. 'The penguin' is a specific group, it is one of the birds that cannot fly, if the bird penguin was used as a general noun (penguins) eliminating the use of 'the' before 'penguin', then Option D would have been correct, but in this case, it's incorrect.