If a novel increases its appeal, then it increases its sales.
A^--->S^
Sentence 2:
If you are a serious novelist, then you care about Lit Style.
SN---->LS
Sentence 3 (Conclusion):
If you are serious novelist, then you are not motivated by sales.
SN---->notS
Now all we need to do is our basic missing premise drills. We need to start with SN, finish with notS. Well, we can fill out the beginning of this chain with:
SN--->LS notS.
Literally all that's left is to draw an arrow between the two disconnected terms. Notice we didn't even need to use sentence 1—don't be suckered into thinking you need to use every piece of the puzzle.
SN--->LS------>notS
The missing term is that second piece. If concerned with Literary Style, then not concerned with Sales.
Let's go find this.
(A): If SN--->notGP? Totally not what we want.
(B): If LS--->notS. Exactly. Click and move on, no need to even waste time reading the rest.