In a gaseous reaction of the type
`aA+bBrarrcC+dD`, which is wrong:

Options

A. a litre of A combines with b litre of B to give `C & D`
B. a mole of `A` combines with `b` mole of `B` to give `C % D`
C. `a g` of `A` combines with `b g` litre of `B` to give `C & D`
D. a molecules of A combines with b molecule of B to give C & D

Answer

To solve the question regarding the gaseous reaction of the type \( aA + bB \rightleftharpoons cC + dD \) and identify which statement is incorrect, we will analyze the options provided.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The reaction involves reactants \( A \) and \( B \) combining to form products \( C \) and \( D \). The coefficients \( a, b, c, \) and \( d \) represent the stoichiometric coefficients of the respective substances.

2. **Analyzing the Options**:
We need to evaluate the statements regarding the quantities of the substances involved in the reaction. The options typically involve volumes, moles, or grams of the substances.

3. **Option A**: "A litre of A combines with B litre of B gives C and D".
- This statement is valid because in gaseous reactions, we can relate volumes to moles directly. According to Avogadro's law, equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. Therefore, this statement is correct.

4. **Option B**: "A mole of A combines with B mole of B gives C and D".
- This statement is also valid. Moles can be directly used in stoichiometric calculations, and the reaction follows the mole ratios given by the coefficients \( a \) and \( b \). Hence, this statement is correct.

5. **Option C**: "A gram of A combines with B gram of B gives C and D".
- This statement is incorrect. The reaction is based on moles, not grams. While it is possible to convert grams to moles using the molar mass, the coefficients in the balanced equation refer to moles, not grams. Therefore, this statement is wrong.

6. **Option D**: "A molecules of A combines with B molecules of B".
- This statement is valid because it refers to the number of molecules, which is consistent with the stoichiometric coefficients in the reaction. Thus, this statement is correct.

### Conclusion:
The incorrect statement among the options provided is **Option C**: "A gram of A combines with B gram of B gives C and D".