Ecologist: Before finding a mate, male starlings decorate their nests with fragments of aromatic plants rich in compounds known to kill parasitic insects.
Conclusion (which suggests)
The function of the decorations is to attract females [not to protect nestlings from parasites].
Evidence
Before finding a mate, male starlings decorate their nests with fragments of aromatic plants rich in compounds known to kill parasitic insects.
Males cease to incorporate such greenery once egg laying starts.
Evaluate
This argument attempts to explain a curious fact: why do male starlings decorate their nests with fragments of aromatic plants?
There are actually two arguments that we need to keep track of. There's an attribution ("Some researchers") which indicates an Opposing Conclusion: the decorations protect nestlings—baby birds—from parasites. And there's some Opposing Evidence for this position indicated by "since": the decorations are made of plants with compounds known to kill parasites.
The pivot indicator "however" tells us we're pivoting away from the Opposing Idea and into the ecologist's argument, which is the argument we're tasked with strengthening. This is the one that we want to strengthen.
Goal
One way to strengthen a rebuttal is to provide evidence against the Opposing Idea. For example, maybe the plant fragments only kill parasites that don't bother starlings.
But, because the stem specifically asks us to strengthen the support for the ecologist's conclusion, we might expect the right answer to support the plausibility of the "attract a female" explanation, showing that this cause (decorating) is associated with the effect (getting the girl).
Because Strengthen questions aren't super predictable, though, we need to read each answer with an open mind and consider how it might impact the argument. Beware answers that strengthen the Opposing Argument made by the researchers!