The most appropriate answer to fill in the blank is – Were.

Well-Preserved- is the most appropriate answer to fill in the blank.

Well-preserved means (of something old) having remained in good condition, showing little signs of ageing.

Let’s first try to learn modals given in the options:

Need = As a modal verb, need is most typically used in negative sentences or in affirmative sentences with a negative meaning. It expresses absence of obligation or necessity, and it is followed by a bare infinitive.

Would = talks about the past; talks about the future in the past; expresses the conditional mood

We also use would for other functions, such as: expressing desire, polite requests and questions, opinion or hope, wish and regrets

Can = It can be used to express ability or opportunity, to request or offer permission, and to show possibility or impossibility.

Must = When we think carefully about facts, we often use "must" to express deductions and conclusions from these.

The first sentence talks about a situation (which is a fact). Therefore, we will use “must” to conclude her situation logically and with certainty.

Hence, option C is the correct answer.

The correct phrase here is clams up which means to refuse to speak or to be unable to speak. In the given sentence, his son refuses to speak, or he didn’t speak every time he feels guilty.

Hence, option C is the correct answer.