Step 1: Understand the meaning of the sentence.
The sentence refers to a philosophy or principle that emphasizes selfless concern for the well-being of others.
Step 2: Examine the meanings of the options.
Polytheism: belief in many gods — unrelated
Altruism: selfless concern for the welfare of others — correct ✔
Iconoclasm: rejection or destruction of traditional beliefs or images — unrelated
Agnosticism: belief that the existence of God is unknown or unknowable — unrelated
Step 3: Choose the best fit.
The word that correctly completes the sentence is altruism.
Step 4: Final conclusion.
The philosophy of putting another’s welfare above one’s own is called \(\boxed{\text{altruism}}\).