Final answer:
The correct phrase to complete the sentence is 'ha sentido triste'. The sentence is in the present perfect tense and Juan is feeling sad about the knee injury.
Step-by-step explanation:
The correct phrase to complete the sentence is ha sentido triste.
The sentence is in the present perfect tense and the subject is Juan. The verb 'sentir' means 'to feel' and in this context, 'sentir triste' means 'to feel sad'.
So the sentence translates to 'Juan has felt sad when hearing about your knee injury'.
Learn more about Completing a sentence in Spanish