To find the power of a biconvex lens, we use the lens maker's formula:$\\ \frac{1}{f} = \left(n - 1\right) \left(\frac{1}{R_1} - \frac{1}{R_2}\right) \\$where:$\\$• $n = 1.5$ is the refractive index of the lens material.$\\$• $R_1 = 20$ cm is the radius of curvature of the first surface.$\\$• $R_2 = -20$ cm is the radius of curvature of the second surface (negative because it is convex).$\\$Substitute the given values into the formula:$\\ \frac{1}{f} = \left(1.5 - 1\right) \left(\frac{1}{20} - \frac{1}{-20}\right) \\$Simplify the expression:$\\ \frac{1}{f} = 0.5 \left(\frac{1}{20} + \frac{1}{20}\right) \\ \frac{1}{f} = 0.5 \left(\frac{2}{20}\right) \\ \frac{1}{f} = 0.5 \times \frac{1}{10} \\ \frac{1}{f} = \frac{0.5}{10} \\ \frac{1}{f} = \frac{1}{20} \\ f = 20$ cm$\\$The power of the lens $P$ is given by:$\\ P = \frac{100}{f}$ (in diopters, when $f$ is in cm)$\\ P = \frac{100}{20} \\ P = 5$ D$\\$Therefore, the power of the lens is $+5$ D, which corresponds to Option 3.