It is asking you to solve for $x$, given that
$$\frac{1}{x} +\frac{1}{a}=\frac{1}{b}.$$
To solve for $x$, first isolate $x$ by itself on one side; for example, move that $\frac{1}{a}$ to the right. That will give you an equation of the form
$$\frac{1}{x} = \text{stuff}.$$
Do the operation on the right, and then take reciprocals (or cross-multiply) to get an expression for $x$ in terms of $a$ and $b$. Then figure out which of the five options given is that expression for $x$.
You can the work as an edit to your question and we can tell you if you are doing it right or not; that will help you learn better than me doing it for you.
(For extra points, figure out exactly on which step you need to assume $a\neq b$...)