Which of the following is correct:
- Only A, B or C is allowed
- Only A, B or C are allowed
- Only A, B and C is allowed
- Only A, B and C are allowed
And if some are correct, do they mean differently?
One rule which seems sensible to me is whether you can replace the list of options with this or these, that is singular or plural replacements. Given your cases, and assuming A, B and C are singular entities you'll get:
- Only this is allowed. OK
- Only this are allowed. Fail
- Only these is allowed. Fail
- Only these are allowed. OK
But, this assumes that your options was singular entities like in "a cat, a dog and/or a bird". If your options were "cats, dogs or birds", then option 1 would fail even if the "or" indicates only one of the options. The usage of plural options , i.e. "cats", does still imply the replacement of "Only these are allowed", making option 2 the correct choice.
- Only cats, dogs or birds is allowed. Fail
- Only cats, dogs or birds are allowed. OK
- Only cats, dogs and birds is allowed. Fail
- Only cats, dogs and birds are allowed. OK
In other words, using "and" always implies plural form (option 4), whilst "or" can imply both forms and depends on whether the option it self is singular or plural. If singular option, like "a cat", use singular form (option 1), or if plural options, like "cats", use plural form (option 2).