Should first introductory paragraph explicitly state the subject of the paper?
The purpose of the introduction is to focus in on the specific, and likely more narrow, topic of your research from a larger perspective. the "larger perspective" is the larger scientific problem to which your study is tied. Therefore you can start the introduction by briefly explaining the larger perspective followed by identifying the existing gaps in knowledge and gradually work towards your own question. 

Many books on scientific writing compare the introduction to a funnel where the wider question is focused to the appropriate width of your research question. the main point is to set your study in a wider perspective so that you can tie your results into the gaps of the larger perspective. This helps readers to get a good perspective of your research and evaluate the results vis-à-vis existing knowledge.

Hence, your scenarios a and b look like a way to describe the writing strategy where in reality the two or three sentences usually requires one or a few paragraphs of text.