Step 1: Meaning of revenue expenditure.
Revenue expenditure is recurring in nature, incurred for day-to-day operations, and its benefit is consumed within the accounting period.
Step 2: Analyse options.
- Purchase of furniture → Capital expenditure (asset creation).
- Payment of salaries → Regular expense, recurring → revenue expenditure.
- Construction of building → Capital expenditure (long-term asset).
- Purchase of machinery → Capital expenditure.
Step 3: Conclude.
Thus, payment of salaries is a revenue expenditure.
Final Answer:
\[
\boxed{\text{Payment of salaries}}
\]