The correct idiomatic expression for traveling by walking is on foot. Expressions like "by foot" or "by walking" are grammatically incorrect in this context.

১. We use "by" for vehicles (e.g., by car, by bus, by train).
২. "On foot" is a fixed phrase that does not change to the plural "on feet."
৩. Grammatically, "on foot" serves as an adverbial phrase of manner.

অতিরিক্ত তথ্য: This is a common trap in English competitive exams; remember: by transport, but on foot.