When does the Strength Social Link become available in P3p?

Answer 1

The strength social link becomes available after the Chariot Level 10 Skit

check this guide for details: http://www.gamefaqs.com/ps2/932312-shin-megami-tensei-persona-3/faqs/50317

Answer 2

To begin the strength social link, you must spend time with Kaz in the sports club to bring your social link up to level 2, and after the sports club, when you talk to Yuko, you must ask if you want to walk home together, The link will then be established.