The strategy of Passive Resistance as advocated by Extremists during the Indian freedom struggle primarily involved the rejection of British rule through non-violent means. This concept aimed to undermine the British colonial authority by leveraging the power of collective non-cooperation and fostering self-reliance. Let's analyze the given options to determine which one aligns best with this strategy:

1. Boycott, Swadeshi and non-cooperation with rulers:
   • The Boycott movement involved refusing to purchase British goods and services, thereby impacting the British economy.
   • The Swadeshi movement encouraged the use of Indian-made goods, promoting self-reliance and economic independence.
   • Non-cooperation included withdrawing from cooperation with the British rulers, refusing to hold any official positions or take part in any official functions.
   • These measures are non-violent and focus on self-sufficiency and non-engagement with the colonial government, aligning well with the principles of Passive Resistance.
2. Assassination of British officers:
   • This method involves violence and direct confrontation, which contradicts the fundamental principle of non-violence in Passive Resistance.
3. Military training of Indian youth abroad:
   • This implies preparation for armed conflict, which does not align with the non-violent nature of Passive Resistance.
4. Armed confrontation with colonial forces:
   • Similarly, armed confrontation is a direct act of violence, which is opposite to the philosophy of Passive Resistance that extremists like Bal Gangadhar Tilak, Bipin Chandra Pal, and Lala Lajpat Rai advocated.

Conclusion: The correct answer is Boycott, Swadeshi, and non-cooperation with rulers, as it involves non-violent resistance and is consistent with the principles of Passive Resistance as advocated by extremist leaders during India's struggle for independence.