In a displacement reaction, a more reactive element will replace a less reactive element. Since hydrogen has medium electronegativity, elements that are more reactive than hydrogen will be more likely to replace it.
Among the options provided:
- Bromine (Br) - this is a halogen and is reactive, but typically reactivity (especially in replacement reactions) tends to decrease going down the group.
- Helium (He) - as a noble gas, helium is very unreactive and cannot participate in displacement reactions.
- Lithium (Li) - this is an alkali metal and is very reactive. Alkali metals are known to easily displace hydrogen from compounds.
- Oxygen (O) - while oxygen is quite reactive, it doesn't usually replace hydrogen in the typical sense of a displacement reaction.
Given these options, the element most likely to replace hydrogen in a displacement reaction is:
lithium (Li).