Step 1: Think of growth rate as a function of frequency.
Imagine plotting the growth rate of variant one against its own frequency in the flask, from nearly $0$ to nearly $1$. The question tells us this growth rate is high whenever the frequency is low, and falls as the frequency rises.
Step 2: Read what that downward-sloping relationship means.
A fitness curve that goes down as frequency goes up means being common actually hurts a type, while being rare helps it. That is precisely the definition used for the "negative" case in frequency dependent selection, since the fitness effect moves in the opposite direction to the frequency.
Step 3: Contrast with the "positive" case.
If growth rate instead rose alongside frequency, common types would keep getting fitter and rarer types would keep losing out until one type disappeared. That upward-sloping relationship is positive frequency dependence, the reverse of what is described here.
Step 4: Note the stabilizing outcome.
Because the rare type always gets the advantage in this flask, neither variant can be driven to extinction. Any drop in frequency for one variant is met with a rise in its own growth rate, pulling the mix back toward balance.
Step 5: Eliminate sexual and plain positive selection.
Sexual selection involves mate competition or mate preference, which is not mentioned at all here. Plain positive selection just means a trait is favored, without saying anything about how that favor changes with frequency, so it is too vague to match this specific pattern.
Step 6: Conclude.
\[ \boxed{\text{Negative frequency-dependent selection}} \]