To solve this problem, we need to understand how the focal length of the objective lens affects the magnifying power of a microscope and a telescope.$\\$1. For a microscope:$\\$The magnifying power $M$ of a compound microscope is given by:$\\ M = \left( \frac{L}{f_o} \right) \left( \frac{D}{f_e} \right) \\$where $L$ is the tube length, $f_o$ is the focal length of the objective lens, $\\ D$ is the least distance of distinct vision, and $f_e$ is the focal length of the eyepiece.$\\$• As $f_o$ increases, $\frac{L}{f_o}$ decreases, leading to a decrease in the magnifying power of the microscope.$\\$2. For a telescope:$\\$The magnifying power $M$ of an astronomical telescope is given by:$\\ M = \frac{f_o}{f_e} \\$where $f_o$ is the focal length of the objective lens and $f_e$ is the focal length of the eyepiece.$\\$• As $f_o$ increases, $\frac{f_o}{f_e}$ increases, leading to an increase in the magnifying power of the telescope.$\\$Based on the above analysis:$\\$• Increasing the focal length of the objective lens decreases the magnifying power of a microscope.$\\$• Increasing the focal length of the objective lens increases the magnifying power of a telescope.$\\$Therefore, the correct option is:$\\$Option 4: Microscope will decrease but that of telescope will increase.