When a pyrimidine is substituted by a purine and vice versa, this type of mutation is known as

✅ Explanation:
In DNA, there are two types of nitrogenous bases: purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil in RNA).
-A transition mutation occurs when a purine is replaced by another purine, or a pyrimidine is replaced by another pyrimidine.
-A transversion mutation occurs when a purine is replaced by a pyrimidine, or vice versa.  

📌 Other Options Explanations:
-(a) Transition: While a type of point mutation, it involves the substitution between the same category of bases (purine-purine or pyrimidine-pyrimidine), not across categories.
-(c) Non sense mutation: This type of mutation results in a premature stop codon, leading to a truncated, non-functional protein.
-(d) Miss sense mutation: This type of mutation results in a change in one amino acid in the protein sequence.

🔑Key Points:
-Transversional mutants: When in the DNA molecules, one purine-pyrimidine base pair is replaced by another purine-pyrimidine base pair, it is called a transversional mutation. This type of mutation involves a change from a purine to a pyrimidine or from a pyrimidine to a purine. For example, an adenine (A) being replaced by a thymine (T) or a guanine (G) being replaced by a cytosine (C).
-Transitional mutants: This type of mutation involves a purine being replaced by another purine or a pyrimidine being replaced by another pyrimidine. For example, adenine (A) being replaced by guanine (G) or cytosine (C) being replaced by thymine (T). 
-Insertion mutants: This type of mutation involves the addition of one or more nucleotide base pairs into the DNA sequence. This can cause a shift in the reading frame of the genetic code, potentially leading to significant changes in the protein produced. 
-Deletion mutants: This type of mutation involves the removal of one or more nucleotide base pairs from the DNA sequence. Like insertions, deletions can also cause a frameshift, leading to potentially harmful effects.