Step 1: Understanding the Concept:
A 'device of investment' refers to an institution, instrument, or channel through which individuals can invest their money with the expectation of generating a return.
Step 2: Detailed Explanation:
- Post Office: Post offices in many countries, including India, offer various government-backed savings and investment schemes like National Savings Certificates (NSC), Kisan Vikas Patra (KVP), and Public Provident Fund (PPF). They are a major channel for investment.
- Bank: Banks are primary financial institutions that offer a wide range of investment products such as Fixed Deposits (FDs), Recurring Deposits (RDs), and mutual funds.
- Market: While one invests 'in the market' (e.g., stock market), the term 'market' itself is a broad concept, not a single device or institution like a bank or post office.
Since both Post Offices and Banks are key institutions that provide investment instruments, 'Both (A) and (B)' is the correct answer.
Step 3: Final Answer:
Both the Post Office and the Bank serve as devices or institutions for investment.