Answer:
(a) several focus groups of their customers - qualitative study
A focus groups consists of a group of five to ten participants, who are allowed to discuss an issue or a product for one or two hours.
(b) observation using a hidden camera - qualitative study
The information obtained from the use of the hidden camera is of qualitative nature. Such information consists in the behaviour displayed by the customers that were recorded.
(c) an online survey of 500 customers. - quantitative study.
The online survey gives quantitative, measurable information that can later be expanded into a statistical study, and also used to establish other quantitative variables like probability distributions, or statistical regressions.