What does the "c" add to the word "aquire" when it already sounds correct meaning the verb "to gain; to possess"?
It's because acquire is formed by prepending the prefix ad- to Latin quaerere. Then later on, it anglicised to acquire.
So why the d changed to c?
It's because of a very common phenomenon called Assimilation.
The last consonant of certain prefixes tends to have the same place of articulation as the the first consonant of the root words.
For example, the prefix in- often gets assimilated to im-, il-, ir- etc.
- im- in 'impossible'.
- il- in 'illegal'.
- ir- in 'irregular' etc.
All these prefixes are the different forms of in-.
The same thing happened to acquire, acquit and accept
- ad + quire -> acquire
- ad + quitare -> acquit
- ad + capere -> accept
Compare require (re + quaerere), the prefix re- does not end with a consonant so it doesn't change.