The most coherent choice to complete this text is 'In addition', as it highlights another way in which Nina pursues her passion for reading, complementing the information about her existing collection. The remaining options, 'Therefore', 'On the contrary,' and 'Nevertheless,' do not effectively convey an expansion on the presented theme: 'Therefore' suggests a conclusion or cause-effect relationship, while both 'On the contrary' and 'Nevertheless' introduce opposing viewpoints or contradiction.

'Nonetheless' is the most suitable option, as it presents a contrasting point (higher prices of organic foods) to the previous information about the growing popularity of organic farming. The other choices — 'Similarly,' 'As a result,' and 'For this reason' — do not create the necessary contrast: 'Similarly' implies agreement, 'As a result' suggests a cause-effect relationship, and 'For this reason' also denotes a causal link.

'As a result' is the optimal choice for this context because it establishes a cause-effect relationship between the strengths of solar energy and the continued research and development endeavors to enhance its potential. The other options — 'On the other hand,' 'In turn,' and 'Once again' — do not effectively denote this relationship: 'On the other hand' introduces contrast, 'In turn' conveys a sequential or reciprocal effect, and 'Once again' repeats a previous statement or occurrence.

'In fact' is the best choice because it conveys that the following information supports and emphasizes the role of exercise in mental health. The other alternatives, 'On the other hand,' 'Despite,' and 'Regardless,' do not aptly communicate the reinforcing nature of the following statement: 'On the other hand' suggests contrast, 'Despite' indicates opposition, and 'Regardless' implies that the following statement is true in spite of something else.

The most appropriate completing choice in this instance is 'Otherwise,' as it conveys the consequence of exceeding moderate consumption of dark chocolate. The other alternatives — 'Similarly,', 'In conclusion,', and 'For this reason' — do not express the intended cause-effect relationship: 'Similarly' indicates agreement between two notions, 'In conclusion' marks the end of a discussion, and 'For this reason' introduces a justification for a preceding statement.