Step 1: Check each option against the requirement of a homogeneous product with no branding. Chocolate and soft drinks are heavily branded and differentiated, so buyers do care which firm they buy from.
Step 2: Gold trade involves fewer large dealers and is influenced by big players, so it does not fit as neatly.
Step 3: Farming has countless independent producers selling an undifferentiated commodity, which is the textbook case of perfect competition.
\[\boxed{\text{Farming}}\]