Answer:
C. ADRs
Step-by-step explanation:
In principle both B. ETFs that include foreign stocks. and C. ADRs are close options to this questions. US firms can buy foreign stock through ADRs i.e. American Depository Receipts. Another way could be through ETF but ETF will track the index of foreign stocks. However, it won't give the ownership of the stock. Hence ADR is a suitable option.