Final answer:
The correct answer to complete the sentence is 'can' as it expresses a general possibility or capability that winters can be really cold sometimes.
Step-by-step explanation:
In the sentence "Winters there ________ be really cold sometimes," the correct modal verb to complete the sentence is can. This is because the statement is talking about a general possibility that is true at any time, and the modal verb 'can' is used to express possibility or capability. The other modal verbs, 'could', 'may', and 'might', also express possibility but in slightly different contexts. 'Could' is often used for past ability or in conditional sentences, 'may' is more formal and less certain, and 'might' implies a lower probability.