Are you given a Fraunhofer spectrum to look at? If you are, then pick out the three thickest lines. If there's multiple thick lines just pick three.
If you're not given a spectrum, then here's what I would use based on my searches:
Visible Fraunhofer spectrum -
http://en.wikipedia.org/w/index.php?...nes.svg&page=1
From this I would say D, E, and F are the most intense lines. Or at least they appear in the regions with the most contrast.
Wavelength vs. Intensity ("strength") graph -
http://en.wikipedia.org/w/index.php?...sky.svg&page=1
D, E, and F again show the most intensity of the labeled peaks.
EDIT: turns out intensity refers to the intensity of the sunlight and not the actual line absorptions. If you're not given a spectrum of your own then I would just ignore this tidbit because you shouldn't be expected to know this without reference material.
From here I would use a reference chart for what elements produce what wavelength, as found here -
http://en.wikipedia.org/wiki/Fraunhofer_lines
Note that several elements contribute to both the D and E lines.
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