Taxonomical aid for identification of plants and animals based on similarities and dissimilarities is a Key.

Key:
A key is a taxonomic aid used for identifying plants and animals based on their similarities and differences. It is a set of questions or statements that lead the user to the correct identification of the organism. Keys are often presented in dichotomous form, where each question or statement offers two alternative choices that direct the user to the next question or statement until the correct identification is made.

Uses of Key:
A key is a useful tool for scientists, naturalists, and hobbyists who need to identify organisms in the field or laboratory. Keys are used in many fields of biology, including botany, zoology, ecology, and microbiology. They are also used in agriculture, forestry, and conservation to identify pests, pathogens, and endangered species.

Advantages of Key:
Keys have several advantages over other taxonomic aids, including:

1. Objectivity: Keys are based on observable characteristics that can be objectively measured and recorded.

2. Efficiency: Keys are designed to quickly and accurately identify organisms, often within a few steps.

3. Flexibility: Keys can be adapted to different taxonomic groups and regions, allowing for a wide range of applications.

4. Accessibility: Keys can be used by anyone with basic knowledge of the taxonomic group, making them accessible to students, citizen scientists, and non-specialists.

Conclusion:
Therefore, the correct answer is option B, a key, which is a taxonomic aid used for identification of plants and animals based on the similarities and dissimilarities.