Theory of errors and adjustments is a fundamental concept in the field of surveying and geomatics. It involves minimizing the effects of various errors that can occur during the measurement and adjustment processes. The goal is to improve the accuracy and reliability of surveying data.

The correct answer to the question is option 'D', which is personal and accidental errors. Personal errors refer to mistakes made by the surveyor due to personal limitations, such as misreading an instrument or recording incorrect data. Accidental errors, on the other hand, are unpredictable and random errors that can occur during the measurement process, such as environmental factors or equipment malfunctions.

Now let's discuss the other options and why they are not the correct answers:

- Option 'A', instrumental errors, refers to errors that occur due to the limitations or inaccuracies of the surveying instruments used. These errors can include misalignment, imperfect calibration, or mechanical imperfections. While instrumental errors are certainly an important consideration in surveying, they are not the main focus of the theory of errors and adjustments.

- Option 'B', mistakes, refers to errors that occur due to human error or carelessness. These mistakes can include misreading measurements, transcribing errors, or calculation errors. While mistakes are certainly a concern in the field of surveying, they are not the main focus of the theory of errors and adjustments.

- Option 'C', systematic errors, refers to errors that occur consistently in the same direction or manner. These errors can be caused by factors such as incorrect instrument calibration, imperfect measurements, or biased observations. While systematic errors are an important consideration in surveying, they are not the main focus of the theory of errors and adjustments.

In summary, the theory of errors and adjustments primarily deals with minimizing the effects of personal and accidental errors in surveying. By understanding and accounting for these errors, surveyors can improve the accuracy and reliability of their measurements and calculations.