The most effective step for a manufacturer that wants to reduce the human cost of extracting the minerals it uses in its smartphones would be verifying that child labor was not used in mining these minerals.
This action directly addresses ethical concerns related to labor practices in the mining industry, particularly in countries where child labor and unsafe working conditions are prevalent. By ensuring that its supply chain is free from exploitative practices, the manufacturer can significantly reduce the human cost associated with mineral extraction while also enhancing its corporate social responsibility and brand reputation.
Committing to mining only in the United States or switching materials might not necessarily reduce human costs effectively, and decreasing product prices does not address the ethical concerns surrounding mineral extraction.