Final answer:
In using proper nouns as a one-thought modifier before a noun, a hyphen is used. 'Smith-Jones' is likely to be the correct context for a one-thought modifier, indicating a joint effort, and 'London-Paris' could be used for something that involves both cities. Context is essential in determining when to use hyphens in such situations.
Step-by-step explanation:
When using two proper nouns as a one-thought modifier before a noun, they should be connected with a hyphen to clarify that they function as a single modifier. An example of this would be 'Smith-Jones' when it is used to describe something like a 'Smith-Jones initiative', meaning an initiative led or created jointly by Smith and Jones. Here, 'Smith-Jones' acts as a compound adjective to modify 'initiative'.
However, in the options provided, Smith-Jones seems the most likely context in which a hyphen might be used as a one-thought modifier, such as in 'Smith-Jones family' or 'Smith-Jones partnership', where two family or business names are combined to indicate a joint effort or ownership. 'London-Paris' could be another potential option if referring to something that pertains to both cities, such as a 'London-Paris train route'.
'Mountain-Valley' and 'Ocean-Sea' are less likely to be used as one-thought modifiers because they generally refer to separate geographical features rather than a combined one or a partnership between the two. It is important to note that context is key in determining the correct use of a hyphen with modifiers.