Final answer:
The best players in the tournament are 'they', and Jen, 'whose' latest short story was published, is only 15 years old.
Step-by-step explanation:
The correct choice of words for the first blank would be 'they' because it is the subject case pronoun that fits the context of the sentence. The sentence should be: 'We must admit that the best players in the tournament are they'.
For the second blank, the correct choice would be 'whose' because it is the possessive case pronoun that is needed to indicate that the short story belongs to Jen. The completed sentence should read: 'Jen, whose latest short story was published last week, is only 15 years old'.