If condition A is matched, condition B needs to be matched in order to do action C

Your first step in these kinds of problems is always to make a logic table.

A | B | Result
-------------------
T | T | do action C
T | F | ...
F | T | do action C
F | F | do action C

Once you've made the table, the solution is clear.

if (A && !B) {
  ...
}
else {
  do action C
}

Do note that this logic, while shorter, may be difficult for future programmers to maintain.

You have two options:

1. Write a function that performs "action C".

2. Rearrange your logic so that you do not have so many nested if statements. Ask yourself what conditions cause "action C" to occur. It looks to me like it happens when either "condition B" is true or "condition A" is false. We can write this as "NOT A OR B". Translating this into C code, we get

   if (!A || B) {
       action C
   } else {
       ...
   }
   

To learn more about these kind of expressions, I suggest googling "boolean algebra", "predicate logic", and "predicate calculus". These are deep mathematical topics. You don't need to learn it all, just the basics.

You should also learn about "short circuit evaluation". Because of this, the order of the expressions is important to exactly duplicate your original logic. While B || !A is logically equivalent, using this as the condition will execute "action C" when B is true regardless of the value of A.

You can simplify the statement like this:

if ((A && B) || (!A)) // or simplified to (!A || B) as suggested in comments
{
    do C
}

Otherwise put code for 'C' in a separate function and call it:

DoActionC()
{
    ....
    // code for Action C
}
if (condition A)
{
    if(condition B)
      {
         DoActionC(); // call the function
      }
    else
      ...
}
else
{
   DoActionC(); // call the function
}