I am unfamiliar with your school system I think. Final year of college / university? Do you have a specialization in your major (graphics, machine learning, embedded, networks / security - something like this?)
I’d go with something from the specialization if you have one. If not, just something that interests you.
In what world? 1k lines is a lot… Even a few hundred can take hours if everything is unknown, code is legacy, and naming is bad.
Like if there is a line like this
memcpy(ptr, src, 4 * 6 * sizeof(real));
What’s that 4?
What’s that 6?
Is real a float? A double?? What are we copying, where, why???
This is a line I saw recently. 1k code is huge even if readable.