Which of the following are nitrogen-fixing plants ?

1. Alfalfa
2. Amaranth
3. Chickpea
4. Clover
5. Purslane (Kulfa)
6. Spinach

Select the correct answer using the code given below:

The correct option is 1, 3 and 4 only.

Explanation:
Nitrogen-fixing plants are those that have a symbiotic relationship with nitrogen-fixing bacteria (such as Rhizobium) residing in their root nodules. These bacteria take gaseous nitrogen from the atmosphere and convert it into a organic form (ammonia) that the plant can readily use. This biological process is primarily associated with plants of the legume family (Fabaceae).

Let's analyze the given plants:
1. Alfalfa (Lucerne): It is a perennial flowering plant in the legume family Fabaceae and is a well-known nitrogen-fixing crop. (Correct)
2. Amaranth: It belongs to the family Amaranthaceae. It is a pseudocereal and does not possess nitrogen-fixing capabilities. (Incorrect)
3. Chickpea: It is an annual legume of the family Fabaceae, widely known for fixing atmospheric nitrogen. (Correct)
4. Clover: Clovers belong to the genus Trifolium in the legume family Fabaceae. They are highly efficient nitrogen fixers. (Correct)
5. Purslane (Kulfa): It is an annual succulent in the family Portulacaceae. It does not fix nitrogen. (Incorrect)
6. Spinach: It is a leafy green flowering plant in the family Amaranthaceae and does not fix nitrogen. (Incorrect)

Therefore, only Alfalfa (1), Chickpea (3), and Clover (4) are nitrogen-fixing plants.