Which of the following elements cannot be detected directly in a given organic compound?

The element that generally cannot be detected directly using standard qualitative analysis methods like the Lassaigne's test is oxygen. While carbon and hydrogen are fundamental constituents and other elements like nitrogen, sulfur, and halogens can be detected after the sodium fusion, oxygen is typically detected by the method of difference (calculating its percentage by subtracting the percentages of all other elements from 100) or by specialized quantitative methods, but not a simple direct qualitative test.