The question 'Will they be there?' is asking about a future action, and the correct negative response is 'no, they won't' where 'won't' is the contraction of 'will not'. Option A is correct. Option B incorrectly uses 'there won't' which doesn't agree with the subject 'they'. Option C uses 'their' which is a possessive pronoun, not 'they are'. Option D uses 'their's' which is not a valid contraction.