Select the incorrect pair
(a) Numerical taxonomy-all observable characteristics
(b). Cytotaxonomy-Cytological information
(c). Chemotaxonomy-Chromosome number and structure
(d). Cladistic taxonomy-Origin from a common ancestor

Explanation

To solve the question, we need to analyze each of the pairs given in the options and identify which one is incorrect.

1. **Numerical taxonomy - all observable characteristics**:
- Numerical taxonomy is a method that classifies organisms based on all observable characteristics. This includes morphological, anatomical, and chemical traits. This statement is correct.

2. **Cytotaxonomy - Cytological information**:
- Cytotaxonomy focuses on the cellular structure and cytological information of organisms. This statement is also correct.

3. **Chemotaxonomy - Chromosome number and structure**:
- Chemotaxonomy is the classification of organisms based on the chemical composition of their cells, such as secondary metabolites and biochemical pathways, rather than chromosome number and structure. Therefore, this statement is incorrect.

4. **Cladistic taxonomy - Origin from a common ancestor**:
- Cladistic taxonomy is based on the concept of common ancestry and organizes organisms based on shared derived characteristics. This statement is correct.

Based on the analysis, the incorrect pair is:
**(c) Chemotaxonomy - Chromosome number and structure**.

### Summary of Steps:
1. Analyze the definition of numerical taxonomy.
2. Verify the definition of cytotaxonomy.
3. Check the definition of chemotaxonomy and identify the error.
4. Confirm the definition of cladistic taxonomy.