In order from least to most dense, it must be: C, B, A and D. Seawater with high temperature causes an object to float, while seawater with low temperature sinks or is denser than high temperature. Salinity is also a factor to determine density; the higher the salinity, the denser the water. In terms of depth, it seems that sample D is the deepest part while sample A is the shallowest. This is determined because temperature drops significantly with depth.
The nucleus contains the DNA and acts like a control center of the cell.
The top layer carry out photosynthesis
The nitrogenous bases placed in between the DNA double helix strands, like rungs on a ladder, are positioned there because of the hydrophobic effect and are stabilized by hydrogen bonding. The stability of DNA also relies on the interactions between the hydrogen bonds between bases, hydrogen bonds between bases and surrounding water, and base interaction between neighboring bases.
Hydrogen bonds are able to stabilize the DNA double helix by pairing up with their complementary base pairs (guanine with cytosine and adenine with thymine)
Hope this was able to help you! I went through my answer and bolded the more important parts, and parts more useful for answering the question.