The body size of the individuals in a population of crabs starts decreasing. a geneticist believes that interbreeding with a smaller species may have happened. a marine biologist believes that the crabs may be undernourished. why are both of these options valid scientific hypotheses?

The reason that these are both valid hypotheses is:
Scientists with different educational backgrounds can come to different interpretations of the same data.

The first step of the scientific method is to make an observation and then the second is to identify a problem. The third step, the formation of a hypothesis, is where scientists tend to differ from one another. A hypothesis serves to explain the given observation and answer the problem. The explanation offered by an individual scientists is based on the scientist's background and understanding of a subject.

I’d say a : water molecules

That would be a working theory.