If two thin convex lenses are kept in contact, the focal length of the combination is:

smaller than either focal length

The correct answer is Option (2) → smaller than either focal length

When two thin lenses with focal length $f_1$ and $f_2$ are kept in contact,

$\frac{1}{F_{net}}=\frac{1}{f_1}+\frac{1}{f_2}$

Since, the reciprocal of the combined focal length is greater than the reciprocal of either $f_1$ or $f_2$, the combined focal length is smaller than either $f_1$ or $f_2$.