Here are the definitions matched with their corresponding letters:
a. Version of a trait - (k) a physical characteristic
b. An allele that is only expressed if a dominant is not present - (b) an allele that is only expressed if a dominant is not present (this is a definition for a recessive allele)
c. A cross looking at two traits - (c) a cross looking at two traits (this is known as a dihybrid cross)
d. A cross involving 3 characteristics - (d) a cross involving 3 characteristics (this refers to a trihybrid cross)
e. Having 2 different alleles - (e) having 2 different alleles (this refers to heterozygous)
f. A diagram used to predict the outcome of a cross - (f) a diagram used to predict the outcome of a cross (this is called a Punnett square)
g. An allele that is always expressed - (g) an allele that is always expressed (this is a definition for a dominant allele)
h. An allele that shows up a little - (h) an allele that shows up a little (usually a recessive allele)
I. The father of genetics - (I) the father of genetics (this refers to Gregor Mendel)
j. Having two of the same allele - (j) having two of the same allele (this refers to homozygous)
k. A physical characteristic - (k) a physical characteristic (this matches with the original statement)
l. A cross looking at only one trait - (l) a cross looking at only one trait (this refers to a monohybrid cross)
Please feel free to ask if you need further clarification or assistance!