Choice A is a noun.
Choice C is an adjective that does not fit logically in context. This letter is from a stationery store, so anyone can purchase items there. Something that is "valid" is allowed or permitted.
Choice D is a verb.
The correct answer is B. An adjective is necessary. A "valued customer" is one who shops somewhere often.
Explanation 2:
Choice A is a preposition that indicates going from one end to another. (I went through a tunnel.)
Choice C is a preposition used when one is passing something to reach another thing. (I went over the bridge.)
Choice D is a preposition used with times or specific venues. (I went at ten o'clock. We met at the mall.)
The correct answer is B. The preposition "on" is used with geographic places such as "island" or mountain.
Explanation 3