The correct answer is C) the tubing and its fittings cannot be properly installed and tightened. The proper installation and tightening of hydraulic tubing and fittings is crucial to prevent leaks and ensure safe operation. If the tubing and fittings are not installed correctly, they cannot be properly tightened, which can lead to leaks when the system is under pressure. This is the main reason why the hydraulic tubing installation shown in figure "D" is incorrect and will likely leak. The other options are incorrect because they do not directly address the issue of improper installation and tightening. Option A describes potential tubing behavior under pressure, but does not explain why the installation is incorrect. Option B relates to overstressing the fittings, but does not explain the root cause. Option D discusses the ability to flex at right angles, but this is not the primary reason for the potential leak.