His family has accumulated wealth over the years.

The correct answer is C. amassed.

To amass is to accumulate or gather a large amount of something, especially money or possessions. In this case, the family has accumulated wealth over the years. This means that they have gradually acquired a large amount of money or possessions.

The other options are incorrect because they have different meanings.

• To drain something is to empty it out or
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  use up all of its contents.
• To squander something is to waste it or spend it foolishly.
• To disperse something is to spread it out or scatter it.

I hope this helps!