The answer is 3.
Families are characterised on the basis of both vegetative and reproductive features of plant species. Tomato (Lycopersicon esculentum) and potato (Solanum tuberosum belong to the same family Solanaceae. 

Maximum Similarity Between Tomato and Potato

1. Species Level:
At the species level, tomato and potato are not similar as they belong to different species - Solanum lycopersicum (tomato) and Solanum tuberosum (potato).

2. Genus Level:
Tomato and potato belong to the same genus, Solanum. This means they share more similarities compared to species level, but they are still distinct species.

3. Family Level:
Both tomato and potato belong to the Solanaceae family, commonly known as the nightshade family. This indicates a closer relationship between the two plants as they share more common characteristics.

4. Kingdom Level:
At the kingdom level, tomato (Plantae) and potato (Plantae) belong to the same kingdom, Plantae, which includes all plants. While this shows a fundamental similarity, it is a broad classification and does not indicate a close relationship between the two.

Therefore, the maximum similarity between tomato and potato will be at the family level (Solanaceae), as they share more common characteristics and traits compared to other levels of classification. Though they belong to different species within the same genus, their family level connection highlights their botanical relationship.

At genus level