What is the appropriate word to complete: I think it costs _____ to fly from London to Edinburgh?

The right choice is "much" because it is commonly used to refer to a large amount or degree of something, particularly with uncountable nouns like "cost" in this context. When discussing expenses, "much" effectively conveys that the cost is significant, which is appropriate when speaking about flight prices.

The other options do not fit well in this particular sentence. "Few" and "many" are used with countable nouns and would not effectively describe the uncountable noun "cost." "Little" suggests a small amount, which does not accurately reflect a likely situation when talking about flying costs between major cities.