Step 1: Understand the blank:
The sentence talks about a retail format built on the idea of 'singularity'. Singularity means dealing in one single line of goods.
Step 2: Recall the retail formats:
A department store sells many lines of goods under one roof. A supermarket sells food and daily needs. A specialty store keeps only one line of goods, such as shoes, books or watches.
Step 3: Match the principle:
Only the specialty store works on the principle of one line, so it is the one that follows singularity.
It offers a deep range and expert advice inside that single line.
Final Answer:
The missing word is Specialty (single-line). Specialty retailing is based on the principle of singularity.