To determine which of the cities mentioned are among the million-plus cities in India based on the data from the Census 2001, we need to consider the population of each city during that time.

A million-plus city refers to a city with a population of one million or more. Let`s evaluate each option:

1. Ludhiana: Ludhiana is a major city in the state of Punjab. According to the Census 2001, the population of Ludhiana was 1,398,467, which means it had a population exceeding one million. Therefore, Ludhiana is one of the million-plus cities in India as of 2001.

2. Kochi: Kochi, also known as Cochin, is a city in the state of Kerala. According to the Census 2001, the population of Kochi was 596,473. Since its population is less than one million, Kochi is not considered a million-plus city in India based on the data from 2001.

3. Surat: Surat is a prominent city in the state of Gujarat. According to the Census 2001, the population of Surat was 2,433,835, surpassing one million. Hence, Surat is one of the million-plus cities in India according to the 2001 census.

4. Nagpur: Nagpur is a city in the state of Maharashtra. According to the Census 2001, the population of Nagpur was 2,405,421, exceeding one million. Consequently, Nagpur is considered a million-plus city in India as of 2001.

Based on the above analysis, the correct answer is Option 4: 1, 2, 3, and 4. Ludhiana, Surat, and Nagpur were all million-plus cities in India according to the data from the Census 2001, while Kochi did not meet the population criterion at that time.