Final answer:
A transmission electron microscope is ideal for resolving structures at the molecular level within cells, but not for observing living cells or studying internal 3D cell structures, for which phase-contrast and confocal scanning laser microscopes would be more appropriate.
Step-by-step explanation:
In the realm of microscopy, a transmission electron microscope (TEM) would be ideal in situation (c) when resolving structures at the molecular level, because it allows for high-resolution imaging of a cell's internal structures. A TEM uses an electron beam that penetrates through the cell to provide detailed images of the components within. This is in contrast to a scanning electron microscope (SEM), which is more suited for observing detailed surface structures of cells due to its mechanism of scanning the cell's surface with a beam of electrons and capturing the reflected electrons to create an image.
However, TEM is not suitable for observing living cells and tissues (a) as the preparation process involves dehydration and embedding the specimen in plastic, which kills the cells. For observing living cells, one may use a phase-contrast microscope. Nor is TEM the best choice for studying internal cell structures in 3D (d); instead, a confocal scanning laser microscope would be more suitable for this purpose as it collects focused light from specific depths within the specimen, which can be reconstructed into a 3D image.
The biological significance of electron microscopes, including TEM, relates to their unparalleled resolution which goes beyond the capabilities of traditional light microscopes. This resolution is essential for studying cell biology, as it allows scientists to visualize components at the molecular level that are invisible under light microscopes.
For viewing thick structures such as biofilms, a confocal scanning laser microscope is particularly useful (from the provided options in question six) because it can scan these dense samples and construct a detailed 3D image layer by layer.