Point mutations means:

The correct answer is Option (1) → Change in one base pair

Option (1) Change in one base pair — Correct. A point mutation arises from a change in a single base pair of DNA. Sickle-cell anaemia, where GAG changes to GUG, is the classical example.

Option (2) Change in one allele — Incorrect. An allele is a whole form of a gene. A point mutation is smaller: it changes one base pair inside the DNA of a gene.

Option (3) Change of base pair at a location without any significance — Incorrect. A point mutation can have a large effect: in sickle cell anaemia one base change replaces glutamic acid by valine in haemoglobin.

Option (4) Addition or deletion of codons in multiple of three at a point — Incorrect. This describes insertion or deletion of whole codons; it removes or adds amino acids without disturbing the reading frame. It is not a change in a single base pair.

NCERT Extract:
Mutation – “… Chromosomal aberrations are commonly observed in cancer cells. In addition to the above, mutation also arise due to change in a single base pair of DNA. This is known as point mutation. A classical example of such a mutation is sickle cell anemia.”