According to the Theory of Consumer Behaviour, Inequality p1x1 + p2x2 ≤ M is called the Consumer’s _______.

The correct option is Budget Constraint.

In the Theory of Consumer Behaviour, a consumer wishes to consume combinations of goods that they can afford given their income and the prices of the goods. Let us assume there are two goods, Good 1 and Good 2, with prices $p_1$ and $p_2$ respectively. Let $x_1$ and $x_2$ represent the quantities of Good 1 and Good 2 consumed by the consumer.

The total expenditure of the consumer on Good 1 is $p_1{x}_1$ and on Good 2 is $p_2{x}_2$. Therefore, the total expenditure on buying both goods is:
$p_1{x}_1+p_2{x}_2$

If the consumer has a fixed income represented by $M$, then the total expenditure cannot exceed this income. This financial limitation is represented by the inequality:
$p_1{x}_1+p_2{x}_2\le M$

This inequality limits or constrains the consumer's choices based on their budget. Hence, it is formally called the consumer's Budget Constraint (while the set of all affordable bundles satisfying this inequality is referred to as the budget set).