Fill in the blank with the most appropriate word.
Excuse me, sir, but may I ask how you came ___________ these goods?
A) After
B) By
C) Out
D) Over

Hint: A preposition is a short, common term that denotes direction (to in "a letter to you"), place (at in "at the door"), or time (by in "by noon"), or that introduces an item (of in "a basket of apples"). The object of a preposition is usually a noun (noon), a noun phrase (the door), or a pronoun (you).

Complete answer:
Here, in this question, we have to find the best option given in the question that fits best in the blank in such a way that it gives correct meaning to the sentence and it should be grammatically correct also.

Let us analyze the given options:
Option A) After – ‘after’ can be used as a preposition, a conjunction and an adverb. Whenever, ‘after’ is used as a preposition it is followed by a noun. For example: I went for a walk after dinner. Therefore, the use of ‘after’ in the blank will make the sentence grammatically incorrect. Therefore, this is an incorrect answer.
Option B) By – preposition ‘by’ is used to show place or location. ‘By’ is considered as the most used and common preposition in framing a sentence. 'by' is added to the verb 'came' to form the phrasal verb 'came by' which means to get something, especially something that is hard to get. So, the use of ‘by’ in the blank is the most appropriate answer and this is the correct answer.
Option C) Out – Preposition ‘out’ with a verb usually expresses the movement that is come and go. The term 'came out' refers to being well-known. So, out is not an appropriate word for the sentence because it can change the essence of the sentence. Therefore, this is an incorrect answer.
Option D) over – Preposition ‘over’ used to talk about the movement or position at higher level than something else. For example: With an umbrella over my head, I was able to protect myself. If we use ‘over’ in the blank then 'Came over' refers to a specific reaction. So, over is not an appropriate word for the sentence. Therefore, this is an incorrect answer.

Hence the correct answer is option ‘B’ i.e, Excuse me, sir, but may I ask how you came by these goods?

Note: The prepositions provided in the options are added to the verb 'came' so as to form different phrasal verbs. Phrasal verbs are basically multi-word verbs. Points to keep in mind for using prepositions:
1) Prepositions are never followed by a verb.
2) It is essential to have a subject and verb and it should be followed by a noun.
3) Avoid using preposition at the end of the sentence.