Final answer:
Lithium (Li) is the strongest reducing agent among the options provided. It is highly reactive and easily donates its electrons, which allows it to reduce other substances, including water.
Step-by-step explanation:
A reducing agent is a substance that donates electrons to another substance, thereby reducing it. The strongest reducing agent will be the one most willing to donate its electrons. The options provided are fluorine gas (F2), fluoride ion (F-), lithium ion (Li+), and lithium (Li). According to standard cell potential tables and chemical reactivity, lithium (Li) is the strongest reducing agent among these options. Lithium metal is known for its ability to easily lose an electron to form Li+, making it highly reactive and an excellent reducer. For example, lithium can even reduce water in a reaction, showcasing its strength as a reducing agent.