Final answer:
To find the position of hydrogen in the activity series, 'Reactivity' is the determining factor. This is because the activity series is based on the ability of elements to displace others in a chemical reaction. The property 'Flammability' is a chemical property while melting point, boiling point, and density are physical properties.
Step-by-step explanation:
To determine the exact position of hydrogen in the activity series, the most relevant factor is b) Reactivity, which refers to the ability of a substance to engage in chemical reactions. The activity series is a chart that ranks elements according to their reactivity, specifically their ability to displace other elements in a chemical reaction. Conductivity, melting point, and density are all physical properties that do not directly determine where an element stands in the activity series.
When considering which property is a chemical property, the options flammability, melting point, boiling point, and density are given. Flammability is the correct answer regarding a chemical property, as it pertains to the substance's ability to undergo combustion. Melting point, boiling point, and density are physical properties related to the substance's state changes and measure of mass per volume, but they do not affect the chemical reactivity criterion used in the activity series.