To solve this problem, we need to determine which statements can be explained by Mendel's Law of Dominance. Let's analyze each statement:$\newline \bullet$A. Out of one pair of factors one is dominant and the other is recessive.$\newline$ - This directly describes Mendel's Law of Dominance, which states that in a pair of contrasting traits, one trait is dominant and masks the expression of the other, which is recessive.$\newline \bullet$B. Alleles do not show any expression and both the characters appear as such in $F_2$ generation.$\newline$ - This statement is incorrect in the context of Mendel's Law of Dominance, as it suggests co-dominance or incomplete dominance, not the classical dominance described by Mendel.$\newline \bullet$C. Factors occur in pairs in normal diploid plants.$\newline$ - This is related to Mendel's Law of Segregation, which states that alleles occur in pairs and segregate during gamete formation. While related, it is not specifically about dominance.$\newline \bullet$D. The discrete unit controlling a particular character is called factor.$\newline$ - This is a general statement about Mendelian genetics, referring to what we now call genes. It is not specifically about dominance.$\newline \bullet$E. The expression of only one of the parental characters is found in a monohybrid cross.$\newline$ - This is a direct consequence of Mendel's Law of Dominance, where the dominant trait is expressed in the $F_1$ generation.$\newline$Now, let's identify which statements are explained by Mendel's Law of Dominance:$\newline$A and E are directly explained by the Law of Dominance.$\newline$Therefore, the correct option is:$\newline$Option 4: A, C, D and E only.