Step 1: Define the paradox.
The Paradox of Thrift, proposed by Keynes, states that when individuals collectively try to save more, total savings in the economy may not rise.
Step 2: Reasoning.
- Higher savings → lower consumption → lower aggregate demand.
- Lower demand reduces output and income → total savings do not increase.
Step 3: Eliminate other options.
- (A) Multiplier → explains how investment increases output by multiple.
- (C) Deficient demand → general situation of insufficient demand.
- (D) Investment → refers to capital expenditure, not paradox.
Final Answer:
\[
\boxed{\text{Paradox of Thrift}}
\]