The correct answer is option 'A': Fluorescent microscope.

Fluorescent microscopy is a technique commonly utilized in various scientific fields, including biology, medicine, and materials science. It allows researchers to visualize and study the structure and function of cells and tissues with high specificity and sensitivity.

Below, we will discuss in detail why fluorescent microscopy is the correct answer and how it is used in various applications.

1. Fluorescence Principle:
Fluorescence is a phenomenon where certain molecules, called fluorophores, absorb light at a specific wavelength and then emit light at a longer wavelength. This emission of light is known as fluorescence.

2. Fluorescent Microscope:
A fluorescent microscope is an optical microscope equipped with specialized filters and light sources to excite fluorophores and capture the emitted fluorescence. It consists of the following components:

- Light Source: Fluorescent microscopes typically use a mercury or xenon arc lamp or LEDs as a light source. These light sources emit light at specific wavelengths that can excite the fluorophores.

- Excitation Filter: An excitation filter is placed in the light path to allow only the desired excitation wavelength to pass through. This helps in selectively exciting the fluorophores.

- Dichroic Mirror: A dichroic mirror, also known as a beamsplitter, is used to reflect the excitation light towards the sample and transmit the emitted fluorescence towards the detector.

- Objective Lens: The objective lens focuses the excitation light onto the sample and collects the emitted fluorescence.

- Emission Filter: An emission filter is used to selectively transmit only the emitted fluorescence towards the detector, blocking the excitation light and any other unwanted background noise.

- Detector: The detector, such as a CCD camera or a photomultiplier tube (PMT), captures the emitted fluorescence and converts it into an electrical signal, which can be further processed and visualized.

3. Applications of Fluorescent Microscopy:
Fluorescent microscopy has a wide range of applications, including:

- Cell Biology: Fluorescent dyes and probes can be used to label specific cellular structures, such as DNA, proteins, or organelles, allowing researchers to study their localization, dynamics, and interactions.

- Immunofluorescence: Antibodies labeled with fluorophores can be used to detect and visualize specific antigens in tissues or cells. This technique is commonly used in immunohistochemistry and immunocytochemistry studies.

- Live Cell Imaging: Fluorescent proteins, such as green fluorescent protein (GFP), can be genetically encoded or introduced into cells to visualize dynamic processes in real-time, such as cell division, protein trafficking, or intracellular signaling.

- Pathology and Diagnosis: Fluorescent microscopy can be used for the detection and analysis of disease markers, such as cancer biomarkers, infectious agents, or genetic abnormalities, aiding in the diagnosis and prognosis of diseases.

- Materials Science: Fluorescent microscopy techniques, such as confocal microscopy and super-resolution microscopy, are used to study the properties and behavior of materials, nanoparticles, and thin films at the nanoscale level.

In conclusion, fluorescent microscopy is a versatile technique used in various scientific disciplines for studying cells, tissues, and materials. It offers high sensitivity, specificity, and spatial resolution, making it a valuable tool for researchers and diagnosticians.