Step 1: Understanding the Concept:
Evaluating correct adjective-preposition collocation for describing financial interest rates.
Step 2: Detailed Explanation:
In standard English grammar, interest rates are described using the scalar adjectives 'high' or 'low' (not 'less' or 'smaller'), governed by the preposition 'at'.
The grammatically correct and natural idiomatic expression is 'at very low rates of interest'.
Step 3: Final Answer:
Hence, 'at very low' is the correct replacement phrase.