In the context of Indian news in recent times, MCX-SX refers to a stock exchange. The correct option is Option 3: Stock exchange.

MCX-SX, also known as the Metropolitan Stock Exchange of India Limited (formerly known as MCX Stock Exchange Limited), is a prominent stock exchange in India. It is recognized as the third recognized stock exchange in the country, after the Bombay Stock Exchange (BSE) and the National Stock Exchange of India (NSE).

Here`s some additional information to support this answer:

1. History and Establishment: MCX-SX was established in 2008 and obtained recognition as a stock exchange from the Securities and Exchange Board of India (SEBI) in 2012. It was established with the aim of providing a transparent and efficient platform for trading in multiple asset classes, including equities, debt instruments, currencies, and interest rate derivatives.

2. Operations and Trading: MCX-SX facilitates trading in various financial instruments, including stocks, equity derivatives, currency futures, and interest rate futures. It operates using a fully automated trading system and provides a robust infrastructure for market participants.

3. Market Reach: While the BSE and NSE are more widely known and have a longer history, MCX-SX has made significant strides in expanding its market reach. It has created a presence in both urban and rural areas, aiming to provide accessible trading opportunities to a wider range of investors. MCX-SX has also focused on enabling small and medium enterprises (SMEs) to raise capital through its platform.

4. Regulatory Compliance: Like any stock exchange, MCX-SX operates under the regulatory framework established by SEBI, India`s securities market regulator. It must adhere to various rules and regulations related to trading, listing, disclosure, and investor protection.

Considering these factors, it is clear that MCX-SX is a stock exchange and not a supercomputer (Option 1), a moon impact probe (Option 2), or a nuclear-powered submarine (Option 4).