The best answer is “C. Chemicals”.
A microscope is of little use for examining a bloodstain. The blood cells are broken down, and there is nothing in a swab of a stain to visualize that would be helpful.
A chromatograph can be useful, but is only rarely used, and never employed in the early stages of bloodstain analysis.
The most useful immediate tests for identifying bloodstains are various chemical tests, some of which are discussed below.
Computers, while helpful for recording and sorting information, are not in and of themselves used in blood stain analysis.
Tests for bloodstains are classified in terms of their specificity as follows:
Visual Test: Does the stain resemble dried blood?
Presumptive screening: The scientist uses chemicals that react with normal constituents of blood, such peroxidase enzymes. An example is the Kastle-Meyer test in which a drop of phenolphthalein is added to the stain, followed by a drop of hydrogen peroxide. If the stain is blood, it will contain peroxidase and cause the phenolphthalein to turn pink. The Kastle-Meyer test and others are explained in the first reference.
Confirmatory tests: These usually involve reacting the stain with chemicals that will cause hemoglobin in the blood to crystallize. The crystals are found by microscopic examination. True, this test uses the microscope, but not in a manner that would make “microscope” the correct answer to your question.
Species tests: Once the presumptive and confirmatory tests are found to be positive (i.e. the stain is definitely blood), it is necessary to prove human origin. The species tests make use of human antibodies that will react only with human antigens (such as human hemoglobin).
A chromatograph is occasionally used to examine a bloodstain. Chromatography is a very sensitive laboratory technique for detecting and identifying a vast array of chemicals in small concentration. The procedure allows identification of chemicals in the stain that are components of blood. As said above, however, the chromatograph is typically used neither early nor commonly at all in bloodstain analysis.
The second reference is a comprehensive PowerPoint presentation on bloodstain analysis.