Non-symbiotic nitrogen fixers are
A) Azotobacter
B) Pseudomonas
C) Soil Fungi
D) Blue-Green Algae

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Hint: Xylem is one of the transportation systems of plants which is used by the plants to transport water to all the parts of the plant so that they can perform the necessary functions.

Complete answer:
Nitrogen fixation is the process of conversion of atmospheric nitrogen into fixed nitrogen. There are certain nitrogen fixing bacteria which convert the atmospheric nitrogen into a form that can be used by the plants. These bacteria can be free living (non-symbiotic) or non-free living (symbiotic).

Symbiotic bacteria live in soil in the association with plants and provide them with fixed nitrogen in return of nutrients, food and shelter for their growth.

Let us analyze the options to find out the correct option.
Option A: Azotobacter is a free living nitrogen fixer which converts atmospheric nitrogen to fixed nitrogen. Therefore, this is the correct option.

Option B: Pseudomonas are gram negative bacteria which causes infection in organisms. They are involved in the process of nitrification which is a process in which the nitrates are broken down to form nitrogen gas. They are usually found in damp areas. Therefore, this is the incorrect option.

Option C: Soil Fungi are microscopic plant-like cells which grow into long threadlike structures that form a mass called mycelium. The mycelium of these fungi absorb nutrients from the soil. They are not involved in the nitrogen fixation process. Therefore, this is the incorrect option.

Option D: Blue-Green Algae phylum consists of both free living bacteria and endosymbiotic plastids present in autotrophic eukaryotes. They fix nitrogen from the atmosphere to fixed forms either in symbiotic or free living form. Therefore, this is the incorrect option.

So, the correct answer is option (A) Azotobacter.

Note: Azotobacter and Cyanobacteria (Blue green algae) fix the atmospheric nitrogen to fixed nitrogen but cyanobacteria has both free living and symbiotic nitrogen fixing forms.