Given:
Quality factor (Q) = 50
Intermediate frequency (fs) = 800 kHz
Local oscillator frequency = 1250 kHz

To find:
Image rejection ratio

Solution:
The image rejection ratio (IRR) is a measure of how well a superheterodyne receiver rejects signals at the image frequency. It is given by the formula:

IRR = Q / (2Q - 1)

Step 1: Calculate the image frequency (fi):
The image frequency is given by the formula:

fi = 2 * fs - fo

where fs is the intermediate frequency and fo is the local oscillator frequency.

fi = 2 * 800 kHz - 1250 kHz
= 1350 kHz - 1250 kHz
= 100 kHz

Step 2: Calculate the image rejection ratio (IRR):
The formula for IRR is:

IRR = Q / (2Q - 1)

Substituting the given value of Q:

IRR = 50 / (2 * 50 - 1)
= 50 / (100 - 1)
= 50 / 99
≈ 0.5051

Step 3: Convert IRR to percentage:
To convert the IRR to percentage, multiply it by 100:

IRR = 0.5051 * 100
≈ 50.51%

Step 4: Calculate the actual image rejection ratio:
The actual image rejection ratio is given by:

Actual IRR = 100% - IRR

Substituting the calculated value of IRR:

Actual IRR = 100% - 50.51%
= 49.49%

Step 5: Calculate the desired image rejection ratio:
The desired image rejection ratio is given by:

Desired IRR = 100% - Actual IRR

Substituting the calculated value of Actual IRR:

Desired IRR = 100% - 49.49%
= 50.51%

Step 6: Convert the desired IRR to dB:
The desired IRR in dB is given by the formula:

Desired IRR (dB) = 10 * log10(Desired IRR)

Substituting the calculated value of Desired IRR:

Desired IRR (dB) = 10 * log10(50.51)
= 10 * 1.7039
≈ 17.039 dB

Therefore, the image rejection ratio when a superheterodyne receiver with a quality factor of 50 is tuned with fs = 800 kHz and a local oscillator frequency of 1250 kHz is approximately 17.039 dB, which is option 'D'.