To solve this problem, we need to find the equivalent focal length of a combination of a convex lens and a concave lens placed in contact.$\\$Given:$\\$• Focal length of the convex lens $= f \\$• Focal length of the concave lens $= -f \\$When two lenses are placed in contact, the formula for the equivalent focal length $(F)$ is given by:$\\ \frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} \\$Substitute the given values:$\\ \frac{1}{F} = \frac{1}{f} + \frac{1}{-f} \\$Simplify:$\\ \frac{1}{F} = \frac{1}{f} - \frac{1}{f} \\ \frac{1}{F} = 0 \\$This implies that $F$ is infinite.$\\$Therefore, the equivalent focal length of the combination is infinite, which corresponds to Option 4.