A natural disaster wipes out most of a population. The allele frequencies of the surviving population may not be the same as the allele frequencies of the original population. This form of genetic drift is called

Bottleneck effect

Explanation:

In bottle neck effect, the amount of genetic variation of a population reduces in lieu to its inability  to adapt to new selection pressure such as climatic change, natural disaster etc. thereby reducing the size of the population by at least one generation.  

For example –  

The genetic variation of Northern elephant seals  is caused by the human inflicted bottleneck i.e excessive hunting

d. in sexual reproduction, offspring have more chromosomes than

either parent.

asexual reproduction results in offspring which are genetically the same as the parent organism (not counting a rare mutation). sexual reproduction produces a random mixture of the genetics derived from both parents

explanation:

the answer to your question is b