Looking at your answer, it appears that your question is actually: what was the best thing you did last week? What is the best thing that happened to me last week? This I can answer: a friend unexpectedly repaid an old debt. This week has not ended and as I can't see the future, I can't tell you what will be the best thing to happen to me during this week.

1) If you wish to emphasize on "Jackson" So far, only Jackson has placed an order. You do not have to use a verbal. A verbal is basically a verb, that is not a verb. It is the form of a verb that can function as a noun, an adjective, or an adverb. There are three types of verbals: - infinitives; - participles; and - gerunds. An infinitive is a verb form preceded by the particle "to". As there are three types of verbals, there are three types of verbal phrases: - infinitive phrases; - participial phrases; and - gerund phrases. An infinitive phrase starts with an infinitive and is followed by objects and/or modifiers. 2) As the writer probably wanted to emphasize on Jackson being "the only person", the writer used an infinitive phrase (to have placed an order) to modify "the only person". The writer used the present perfect tense verb form. So far, Jackson, is the only person to have placed an order.

Check this out http://www.learnenglish.de/grammar/articlestext.htm.

Say for example the official announcement of the interim results of the company is expected to be made in the first week of August. We can use the word "due" instead of "expected". The official announcement of the interim results of the company is due in the first week of August.

For more information http://en.wikipedia.org/wiki/Course_credit.

You say something and then you realize something even better than what you have just said. You would use “better yet” to introduce what you think is even better than what you have just said. Examples I’ll send you an e-mail about this. Better yet, give me your phone number and I’ll call you this evening to discuss it. We should get feedback from all the employees, better yet, let's call all of them in for a meeting and discuss it in detail.

Was the gift precious to you at that time but for some reason it is no longer precious now?

The subject is "Edward". Take out the adjectival phrase "is the only person among all employees who" and you are left with: Edward has to explain () opinion to the board members. So (D) is the wrong answer.