Mendel determined that for a given character(i.e. pea seed shape) an individual has two factors that these can exist in two forms (R or r). Today we would describe R and r as being:. a. different alleles of the seed shape gene.
b. different genes of the seed shape locus.
c. different genotypes of the seed shape character.
d. different loci of the seed shape gene.

a. different alleles of the seed shape gene.

Explanation:

Mendel crossed different varieties of pea plants and he observed how phenotypic traits passed to the progeny. From these experiments, Mendel formulated the 'First Law of Segregation', where he observed that traits may exist in pairs that segregate (separate) at meiosis. During meiosis, i.e., gamete formation, these two factors separate from each other, thereby each gamete has the same probability of receiving either factor. Nowadays, we know that these two factors represent two different gene variants or 'alleles' for a given gene locus. Alleles can be classified into dominant or recessive as in the example above described, where the R factor (round) dominates on the r factor (wrinkled) to determine the seed shape.

C) by the diffusion of molecules across the cell membrane