The correct manner to connect batteries, particularly in a typical setup (such as jump-starting a car), is:
b. Positive to positive and negative to negative
This means you connect the positive terminal of the first battery to the positive terminal of the second battery, and the negative terminal of the first battery to the negative terminal of the second battery. If you're connecting to a vehicle's engine block (often done for grounding), the best practice is to connect the negative terminal of the second battery (or the last connection) to a grounded metal part of the engine block instead of directly to the negative terminal of the first battery, which is considered in the last option. This helps to reduce the risk of sparks near the battery.
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