Explanation:

TISCO, which stands for Tata Iron and Steel Company (now known as Tata Steel), is a prominent example of a private sector undertaking in India. It was founded by Jamshedji Tata and has always been privately owned and managed. Option A, BHEL (Bharat Heavy Electricals Limited), is a public sector undertaking (PSU) owned by the Government of India. Option B, SAIL (Steel Authority of India Limited), is also a major PSU in the steel sector. Option D, OIL (Oil India Limited), is another PSU in the oil and gas industry. Thus, TISCO is the sole private entity among the choices.