ab + c = d
In other words, you can put it like this:
1(ab) + 1c = 1d
Since our answer choices, let's try to get a by itself.
ab + c = d
Subtract both sides by c.
ab = d - c
Now, a is being multiplied by b, so we need to perform the opposite order of operations (divide).
Divide both sides by b.
You're left with:
a = (d - c)/b
Your answer is D.)
I hope this helps!