Lighter-Than-Air Balloon Gas
Puzzle 7 of Wonderful Sights Pack
Question is: Lighter-Than-Air Balloon Gas
and the answer is: HELIUM
Word Meaning:
HELIUM — (uncountable) The second-lightest chemical element (symbol He), with an atomic number of 2 and atomic weight of 4.002602, a colorless, odorless and inert noble gas.
Definitions from Wiktionary.