Black stem rust of Wheat is caused by:
a. Fungus
b. Protozoa
c. Alga
d. Bacterium

Hint: Black stem rust is usually caused by Puccinia graminis, which is the organism that produces significant diseases that affecting the cereal crops, mainly these organisms affect the crop species which includes bread wheat, durum wheat, barley, etc.

Complete answer:
- Puccinia graminis is a fungus which mainly attacks the cereal crops.
- This stem rust disease had affected cereal farming throughout history.
- But from 1950, after the discovery of North Indian wheat plants by prof K.C Mehta, which are of the resistant type of crops.
- And fungicides are also very effective against this stem rust disease.
- This fungus mostly occurs on leaf sheets and also found on stems, leaves, and glumes.
- Where this fungus forms the brick red elongated blister-like pustule.
- Generally, it attacks the parts of the plant that are above ground level.
- The spores are landed on green wheat plants from the pustules which invade the outer layer of bark.
- This fungus causes the yield loss in many ways, which are, they are the fungus where they absorb the nutrients from the growing plants, which are generally useful for growth and development.
- As above said they form the pustules, where they break through the epidermis which disrupts the plant control of transpiration and it may cause the desiccation.
- As also interfere with the vascular tissue and they make the plant weak by making the stem weak.

Hence, the correct answer is option (A).

Note: The fungus which causes the stem rust is an obligate biotroph, where it requires other organisms for growth and nutrition, and their life cycle is also an alternative which means that they require two hosts to complete the life cycle.