The change in the optimal quantity of a good when its price changes and the consumer's income is adjusted so that she can still just afford the original bundle she was buying before the price change is called what?

Introduction / Context:
In microeconomics, understanding how consumers react when prices change is fundamental. Economists split the total effect of a price change into two components: the substitution effect and the income effect. This question specifically describes a situation where the consumer's income is adjusted so that she can still just buy the original bundle after a price change. That phrasing is a classic textbook definition of the substitution effect. Recognising this type of wording is very important for answering theory questions on consumer behaviour correctly.

Given Data / Assumptions:
- The price of a good changes.
- The consumer's income is hypothetically adjusted so that the original bundle remains just affordable after the price change.
- We need to identify the term used for the resulting change in the optimal quantity demanded of that good.
- We assume rational consumer behaviour and standard indifference curve analysis.

Concept / Approach:
When the price of a good changes, two things happen: the relative price between goods changes and the consumer's real purchasing power changes. The substitution effect captures the change in quantity demanded solely due to the change in relative price, holding the consumer at the same utility level by adjusting income so that the original bundle is still just affordable. The income effect then captures the change in quantity demanded due to the change in real income or purchasing power. The description in the question matches exactly the standard definition of the substitution effect given in consumer theory.

Step-by-Step Solution:
Step 1: Focus on the key phrase: the consumer's income is adjusted so that she can still just buy the original bundle after the price change. Step 2: This adjustment of income is a mental experiment used by economists to separate the impact of relative price change from the impact of a change in purchasing power. Step 3: When we keep the consumer on the same indifference curve by this income adjustment, any change in her chosen bundle comes only from the change in relative prices. Step 4: This change in the optimal quantity of the good due purely to the relative price change is called the substitution effect. Step 5: Therefore, the correct term for the phenomenon described is the substitution effect.

Verification / Alternative check:
An alternative way to verify is to recall the usual decomposition: Total effect of a price change = substitution effect + income effect. The hypothetical income adjustment that makes the original bundle just affordable again is the hallmark of the Hicksian definition of the substitution effect. No such adjustment is described when we talk about the law of demand in general form, or about the problem of choice, or about optimal choice by itself. Only the substitution effect is defined in precisely this way, so the match is exact and verifies the answer.

Why Other Options Are Wrong:
Law of demand is wrong because it states the general inverse relationship between price and quantity demanded, not the refined decomposition with income adjustment.
Problem of choice is wrong because it refers broadly to the consumer deciding how to allocate limited income among many goods, and does not specifically describe income adjustments after a price change.
Optimal choice is wrong because it is the final chosen bundle at given prices and income, without isolating the part due to relative price changes alone. The question requires the technical term substitution effect.

Common Pitfalls:
Students often confuse substitution effect and income effect because both arise from a price change. A typical mistake is to answer law of demand whenever a price change is mentioned. The trick in this question lies in the phrase that income is adjusted so that the consumer can still just buy the original bundle. This clause must automatically trigger recognition of the substitution effect. Practising a few numerical and diagram based examples of decomposing total effect into substitution effect and income effect can help to firmly fix the concept in memory.

Final Answer:
The described change in the optimal quantity of a good, when price changes and income is adjusted to keep the original bundle just affordable, is called the substitution effect.