Explanation:

The correct answer is option 'B' - which.

The word "which" is used to introduce a relative clause, providing additional information about the noun or noun phrase that precedes it. In this case, the relative clause is "travels directly to Liverpool," and it describes the train that has been canceled.

Here's a breakdown of the options and why they are correct or incorrect:

a) that: "That" can also be used to introduce a relative clause, similar to "which." However, in this sentence, "that" would not be the best choice because it is typically used for essential or restrictive clauses. Since the information about the train traveling to Liverpool is not essential for understanding the sentence, "that" is not the most appropriate option.

b) which: This is the correct answer. "Which" is commonly used for non-essential or non-restrictive clauses. It provides additional information about the noun or noun phrase without changing the core meaning of the sentence. In this case, the relative clause "travels directly to Liverpool" adds extra details about the train without altering the meaning of the sentence.

c) who: "Who" is used to refer to people, not objects or things. Since the sentence is about a train, "who" is not the appropriate choice.

d) whose: "Whose" is used to indicate possession or ownership. It is not suitable in this context because it does not provide the necessary information about the train being canceled.

Therefore, the correct answer is option 'B' - which.