All individuals have two alleles for a given trait. according to mendel's these alleles are passed down one each from both mother and father.
a.) law of segregation
b.) law of inheritance
c.) law of dual alleles
d.) law of independent assortment

The correct answer for this question is A. Law of segregation.

All individuals have two alleles for a  given trait. According to Mendel's Law of segregation, these alleles are passed down one each from both mother and father.  

Explanations;
According to this law of segregation the allele pairs separate or segregate during the formation of gamete, during the process of meiosis, leaving each cell with a single allele for each trait, and randomly unite during fertilization. One pair of allele comes from the mother while the other pair comes from the father, and joins together to form a diploid cell. Therefore, organisms inherit two alleles for each trait one from each parent. 

It explains why rocks appear in layers