Explanation:

Plasmodium is a genus of parasitic protozoans that causes malaria in humans. Its life cycle involves two hosts - a vertebrate host (human) and an invertebrate host (female Anopheles mosquito).

The life cycle of Plasmodium is completed in two phases: the human phase (sexual reproduction) and the mosquito phase (asexual reproduction).

Human Phase:
1. The female Anopheles mosquito injects sporozoites (infective stage) into the bloodstream of a human during a blood meal.
2. The sporozoites travel to the liver and infect hepatocytes (liver cells).
3. Inside hepatocytes, the sporozoites undergo asexual reproduction (schizogony) to form thousands of merozoites.
4. The hepatocytes rupture, releasing the merozoites into the bloodstream.
5. The merozoites invade red blood cells and continue to reproduce asexually, causing symptoms of malaria.
6. Some merozoites differentiate into male and female gametocytes (sexual stage) within the red blood cells.

Mosquito Phase:
1. When a female Anopheles mosquito feeds on an infected human, it ingests the gametocytes along with the blood.
2. Inside the mosquito's midgut, the male and female gametocytes undergo sexual reproduction to form zygotes.
3. The zygotes develop into motile ookinetes, which penetrate the midgut wall and form oocysts.
4. Inside the oocysts, multiple rounds of asexual reproduction occur, resulting in the production of thousands of sporozoites.
5. The oocysts rupture, releasing sporozoites that migrate to the mosquito's salivary glands.
6. When the mosquito feeds on another human, it injects sporozoites into the bloodstream, starting the cycle again.

Temperature Requirement:
The invertebrate host, the female Anopheles mosquito, plays a crucial role in the transmission of Plasmodium. The correct answer is option 'B' - low temperature.

- The low temperature is required for the development of Plasmodium within the mosquito.
- The mosquito's midgut provides a suitable environment for the sexual reproduction and development of the parasite.
- High temperatures inhibit the development of Plasmodium within the mosquito, preventing the completion of its life cycle.
- This is why areas with high average temperatures have lower rates of malaria transmission, as the mosquitoes are unable to support the development of Plasmodium.

Hence, the invertebrate host, the female Anopheles mosquito, is needed to complete the life cycle of Plasmodium due to low temperature requirements.