For the following research problem, which of the listed research methods will…

The research problem asks to study the "perspectives" of mothers of children with disabilities about the principal's influence on trust. The key clue is the word "perspectives" — the goal is to explore people's subjective views, opinions and lived experiences in depth, not to measure variables numerically or to establish a cause-and-effect relationship.

This is a qualitative, exploratory aim. A Focused Group Discussion (FGD) brings together a small group of participants (here, the mothers) and lets a moderator draw out their shared and differing views, feelings and experiences through open, interactive conversation. It is therefore the most appropriate method for capturing such perspectives.

The other listed methods do not fit this aim: the experimental method deliberately manipulates and controls variables to test cause and effect; the descriptive survey method collects standardised, mostly numeric data from a large sample to describe "what is"; and the ex post facto method looks back at an already-existing condition to infer possible causes. None of these is designed to richly explore participants' perspectives, so the FGD-based method is the best choice.