The magnifying power of an astronomical telescope can be decreased by A. Decreasing the focal length of the eyepiece B. Increasing the focal length of the eyepiece C. Increasing the focal length of the objective D. None of these

The correct answer is: A. Decreasing the focal length of the eyepiece.

The magnifying power of a telescope is given by the formula:

$M = \frac{f_o}{f_e}$

where $f_o$ is the focal length of the objective and $f_e$ is the focal length of the eyepiece.

Therefore, the magnifying power of a telescope can be decreased by decreasing the focal length of the eyepiece.

Here is a brief explanation of each option:

• Option A: Decreasing the focal length of the eyepiece will decrease the magnifying power of the telescope. This is because the focal length of the eyepiece is in the denominator of the magnifying power formula.
• Option B: Increasing the focal length of the eyepiece will increase the magnifying power of the telescope. This is because the focal length of the eyepiece is in the denominator of the magnifying power formula.
• Option C: Increasing the focal length of the objective will increase the magnifying power of the telescope. This is because the focal length of the objective is in the numerator of the magnifying power formula.
• Option D: None of these. This is because Option D is not a correct answer.