Nucleotide excision repairs refers to short sequences of DNA being removed and repaired with the undamaged strand. It is usually removing bulky adducts on DNA, mostly thymine dimers which are caused by UV radiation. DNA mismatch repair is fixing a single mismatched DNA. Homologous recombination repair is fixing a double stranded break and using the sister chromosome as a template. These are mostly autosomal recessive disorders. Genomic instability simply refers to the high number of mutation which leads to instabilities in the genome. 

The correct answer here is 21. Remember that we can begin by excluding the option that is 25 because there are only 23 pairs of chromosomes in a human being. If you chose any of the other options, remember that down syndrome is also called trisomy 21. This is a syndrome where the 21st pair of chromosomes has a third mutation on it. 

Nucleotide excision repair corrects DNA that turns into pyrimidine dimers. These are usually caused by ultraviolet radiation and result in sizable DNA adducts. 

Point mutations occur when one nucleic base is swapped for a different nucleic base.

It is an insertion because a nucleic acid was added to the sequence.

A transition mutation represents a purine-to-purine mutation or a pyrimidine-to-pyrimidine mutation. This could be a change of A to T, or vice versa. It could also be a change of C to G, or vice versa. Remember, a transition mutation does not insert or delete any bases. It simply changes a base.

The most deleterious mutation would be a frameshift mutation. This would attained by inserting or deleting multiples of 1 or 2 bases in any important coding regions (exons, splice site, translation start site). Inserting or deleting multiples of 3 bases would not cause a frameshift mutation. A mutation in 3' UTR or intron would essentially have minimal effects. A mutation in the wobble position would most likely not change the amino acid.

Recall that a frameshift mutation occurs when part of one or more codons in lost in such a way that the entire sequence of codons "shifts," making the transcribed mRNA translate as a different pattern of amino acids during protein synthesis. This is a big problem, since it means that a sequence of DNA coding for a specific protein now likely codes for a completely, fundamentally different one.

The correct answer is:

A T G C T G G A T A G C T A

T A C G A C C T A T C G A T

Compare to the original DNA, and note where one base pair was removed:

A T G C C T G G A T A G C T A

T A C G G A C C T A T C G A T

The codons on a piece of mRNA transcribed from the top strand of the non-mutated form would read:

UAC GGA CCU AUC GAU

However, mRNA transcribed from the mutated DNA would read:

UAC GAC CUA UCG AT[ ] (we don't know the last codon, since the next base pair is not given)

This is a serious issue. The non-mutated form codes for these first four amino acids...

tyr  gly  pro  ile

...while the mutated form codes for these first four amino acids:

tyr  asp  leu  ser

Clearly, frameshift mutations can be very dangerous.

The correct answer here is apoptosis. All other options are things that can occur to a cell that has been damaged, however, the only option where a cell will abruptly commit suicide is apoptosis. The other options, such as cancerous cells or dormancy do occur.