In nature, the processes of inbreeding and outbreeding play a crucial role in maintaining genetic diversity within plant populations. These processes are regulated by intraspecific and interspecific incompatibility.

Interspecific incompatibility:
Interspecific incompatibility refers to the inability of closely related species to produce viable and fertile offspring. This occurs due to genetic and physiological differences between species that prevent successful fertilization and development of hybrid offspring. In plants, interspecific incompatibility acts as a reproductive barrier that limits gene flow between species. This mechanism ensures that species remain distinct and do not merge into a single population, promoting biodiversity.

Intraspecific incompatibility:
Intraspecific incompatibility refers to the inability of individuals within the same species to produce viable and fertile offspring. It occurs due to genetic differences between individuals, resulting in self-incompatibility. Self-incompatibility mechanisms prevent self-fertilization and promote outbreeding. This mechanism is important for maintaining genetic diversity within a population and reducing the risks of inbreeding depression.

Self-incompatibility in plants can be controlled by a multi-allelic system known as the S-locus. The S-locus consists of two genes, S-allele and S-RNase, which determine the compatibility between pollen and the stigma of the same plant. Each individual plant possesses two different S-alleles, and if the pollen carries an S-allele that matches either of the two S-alleles of the stigma, self-fertilization is prevented. This mechanism ensures that plants are not fertilized by their own pollen, promoting outbreeding and maintaining genetic diversity within a population.

In conclusion, the regulation of inbreeding and outbreeding of plants is primarily controlled by intraspecific and interspecific incompatibility mechanisms. Interspecific incompatibility prevents crossbreeding between closely related species, promoting species differentiation and biodiversity. Intraspecific incompatibility, specifically self-incompatibility, prevents self-fertilization and promotes outbreeding, maintaining genetic diversity within populations and reducing the risks of inbreeding depression.

Correct option is 'b' if u go through the NCERT then u find this statement as usual ..