Have I ruined my chances of getting into a decent University?

If the D is in a subject requirement class, then you MUST at least retake the course with a C or better. You can be rejected for a single missing subject requirement. But if the D is in a non-required subject, then it matters less.

If the D is in a prerequisite for a particular impacted major at the UC school, then you can also be rejected by the department of the major. Every app is evaluated for general university admissions requirements as well as by the department for a declared major. Impacted majors can have much more stringent requirements than needed for general admission, particularly for engineering or special programs with limited enrollment.

Usually you can correct one D while you're still in HS. You'll always have a second chance to get into the UC of your choice by attending a community college and completing the transfer requirements.

Depends on your other grades. If they are mostly B's an C's then your chances are slightly slimmer. If this D is really an outlier, then I wouldn't worry about it.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.