Among the options you've provided, the most appropriate disadvantage of using a functional language is:
It’s more difficult to execute many operations at the same time.
Functional languages often emphasize immutability and recursion rather than stateful constructs, which can make certain types of concurrent programming more challenging compared to imperative languages. However, it's worth noting that functional languages do have robust models for handling concurrency, but the learning curve can be steeper.
The other options can also have some validity but often depend more on the specific context, programmer experience, or the particular functional language being used.