The tendency of offspring to differ from parents is called variation. This concept is a fundamental principle in the field of biology and genetics.

**Variation and Offspring**
Offspring inherit genetic information from their parents, which determines their traits and characteristics. However, no two individuals are exactly alike, even within the same species. This variation is due to the presence of different combinations of genes, as well as the influence of environmental factors.

**Explanation of the Options:**

a) Variation: This option is the correct answer. Variation refers to the differences between individuals of the same species. It is caused by genetic and environmental factors.

b) Heredity: Heredity is the passing of traits from one generation to the next through genes. It is related to the concept of variation but does not specifically describe the tendency of offspring to differ from parents.

c) Inheritance: Inheritance is the process by which traits are passed from parents to offspring. It is similar to heredity and is not specifically focused on the tendency of offspring to differ from parents.

d) Resemblance: Resemblance refers to the similarity or likeness between individuals. While offspring may share some traits and characteristics with their parents, the tendency to differ is more prominent, making this option incorrect.

**Importance of Variation:**
Variation is crucial for the survival of a species and the process of evolution. It provides the raw material for natural selection, allowing individuals with advantageous traits to survive and reproduce, while those with less beneficial traits may struggle to survive. Over time, this can lead to the development of new species and the adaptation to changing environments.

**Causes of Variation:**
There are two main sources of variation:

1. Genetic Variation: Genetic variation is caused by the presence of different alleles (alternative forms of a gene) within a population. It arises from processes such as mutation, recombination, and gene flow.

2. Environmental Variation: Environmental factors, such as climate, nutrition, and exposure to toxins, can also influence the traits and characteristics of individuals. This type of variation is not inherited but can contribute to the overall variation within a population.

**Conclusion:**
The tendency of offspring to differ from parents is known as variation. It is a fundamental aspect of biology and genetics, providing the basis for evolution and the adaptation of species to changing environments. Understanding variation is essential in fields such as agriculture, medicine, and conservation, as it helps in the selection of desirable traits, the study of genetic diseases, and the preservation of biodiversity.