Aneuploidy refers to the:

The correct answer is Option (2) → Gain or loss of a chromosome

Option (1) Complete doubling of chromosomes — Incorrect. An increase in a whole set of chromosomes is polyploidy; NCERT says it follows failure of cytokinesis after telophase and is often seen in plants. Aneuploidy is the gain or loss of only one or a few chromosomes.

Option (2) Gain or loss of a chromosome — Correct. Aneuploidy is the gain or loss of a chromosome(s), caused by the failure of chromatids to separate during cell division. Down’s syndrome (an extra chromosome 21) is an example.

Option (3) Complete loss of all chromosomes — Incorrect. Losing all chromosomes is not possible in a living cell.

Option (4) Entangling of chromosomes — Incorrect. There is no such condition. Aneuploidy is about the number of chromosomes (a gain or loss), not about chromosomes getting tangled.

NCERT Extract:
Chromosomal Disorders – “… Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. For example, Down’s syndrome results in the gain of extra copy of chromosome 21. Similarly, Turner’s syndrome results due to loss of an X chromosome in human females. Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and, this phenomenon is known as polyploidy. This condition is often seen in plants.”