How many letters does the top answer for “burning which of these metals will produce a bright white flame” have?

The most common answer is magnesium (9 letters).

Crossword clue: burning which of these metals will produce a bright white flame

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

magnesium 9 letters otdb

a light silver-white ductile bivalent metallic element; in pure form it burns with brilliant white flame; occurs natural