The focal length of a convex lens is maximum for which of the following?

In the displacement method,the distance between the object and the screen is $70 \; cm$ and the focal length of the lens is $16 \; cm$. Find the separation between the magnified and diminished image positions of the lens. (in $; cm$)

In an experiment with a convex lens,the plot of the image distance $(v')$ against the object distance $(u')$ measured from the focus gives a curve $v' u' = 225$. If all the distances are measured in $cm$,the magnitude of the focal length of the lens is $cm$.

$A$ biconvex lens of focal length $f$ and radii of curvature of both surfaces $R$ is made of a material of refractive index $n_{1}$. This lens is placed in a liquid of refractive index $n_{2}$. How will this lens behave?

The power (in diopters) of an equiconvex lens with radii of curvature of $10 \ cm$ and refractive index of $1.6$ is

$A$ convex lens made of glass has a focal length of $0.15 \,m$ in air. If the refractive index of glass is $\frac{3}{2}$ and that of water is $\frac{4}{3}$, what is the focal length of the lens when immersed in water (in $\,m$)?