Final answer:
The element that is most likely to pair with an alkali metal in a 1:1 relationship based on valence trends is fluorine (F), due to its high reactivity and strong electronegativity among the halogens.
Step-by-step explanation:
In considering which element is most likely to pair with an alkali metal in a 1:1 relationship based on valence trends, we need to look at the halogens. The halogens, especially fluorine, chlorine, bromine, and iodine, have seven valence electrons and are thus one electron short of having a complete octet. This makes them highly reactive and eager to gain an electron. Alkali metals, on the other hand, have a single valence electron that they tend to lose to achieve a filled electron shell. When an alkali metal and a halogen combine, the alkali metal donates its electron to the halogen, forming a 1:1 ionic bond. Among the options given, chlorine (Cl), fluorine (F), and bromine (Br) are all halogens. However, fluorine is the most reactive halogen due to its electronegativity and small atomic radius, making it most likely to form a stable 1:1 compound with an alkali metal.