Question

Two convex lenses of focal lengths f₁ and f₂ are mounted coaxially separated by a distance. If the power of the combination is zero, the distance between the lenses is
(a) |f₁ - f₂|
(b) f₁ + f₂
(c) $\frac{f_{1}f_{2}}{|f_{1}-f_{2}|}$
(d) $\frac{f_{1}f_{2}}{f_{1}+f_{2}}$

Answer 1

This looks like a homework problem.
We know that the power is the reciprocal of the focal length in meters. When combining lenses one of these parameters is additive:

focal length

power

If you know when one, then you will easily be able to calculate the answer.
If this hint is not enough to solve the problem, add a comment. I’ll check later.