Explain why a mutation that occurs in a codon may result in a change of an amino acid in the ptotein sequence?

If the mutation occurs in the DNA, then when transcription occurs complementary base pairing in the DNA-RNA heteroduplex results in a different ribonucleotide at the corresponding position in the pre-mRNA and eventually the mRNA that is exported to the cytoplasm to initiate translation on a ribosome. But because complementary base pairing also informs the match between each mRNA codon triplet and the anticodon triplet on a tRNA molecule, and because each aminoacyl tRNA synthetase co-recognizes a specific tRNA and amino acid, a different mRNA codon guarantees that a different tRNA will bind it and this may add a different amino acid to the 3' end of a nascent polypeptide chain.  It is just such a mutation that gives rise to the HbS hemoglobin that is characteristic of sickle cell anemia.  I hope this has been helpful... best wishes.

when the mutatuion occur,the codon will change.the original anticodon cant fix anymore with the codon.different anti codon  will complement with the mutated codon.different anti codon will code for difeerent amino acid.so,there will a change in amino acid sequence in thee protein structure and can lead to the malfunctioning protein..

for example,sickle cell anaemia

Each codon is composed of three bases.  ATG for instance codes for Methionine (abbreviated Met).  If a point mutation occured in the third base of this codon to generate ATA (a G to A transition), then the mutated codon would encode Isoleucine (abbreviated Ile).  There are many examples of this.  Look in your chart and figure it out!