2. The <d> in these twelve words is replaced with another letter because of assimilation. When things assimilate, they get more similar.
Assimilation is a good name for this for two reasons. For one thing, it contains the prefix ad- with the <d> assimilated to an <s>: assimilation = ad+ s + similation. So the word assimilation contains an example of itself!
For another thing, the base simil in assimilation is the same base that is in the word similar. The base simil means “like.” And that is what assimilation is all about: Sounds or letters assimilate when they change to be more like other sounds or letters.
Sounds change to be more like one another in order to make the word easier to say. We could say things like ∗adsist or ∗adcount, but it is easier if the sounds spelled by the <d> change to be like the sound right after them. When the sound changes, we often change the spelling, too. So instead of ∗adsist, we have assist. Instead of ∗adcount we have account. And we say that the sounds and the spellings have assimilated.