The correct answer is B. Nominal scale.
A nominal scale is a scale of measurement that classifies data into categories. The categories are mutually exclusive and exhaustive, and there is no natural ordering of the categories. For example, gender is a nominal scale with two categories: male and female.
An ordinal scale is a scale of measurement that ranks data into categories. The categories are mutually exclusive and exhaustive, and the categories have a natural ordering. For example, a scale of customer satisfaction with five categories: very satisfied, satisfied, neither satisfied nor dissatisfied, dissatisfied, and very dissatisfied.
An interval scale is a scale of measurement that ranks data into categories and has equal intervals between the categories. For example, a scale of temperature in degrees Celsius.
A ratio scale is a scale of measurement that ranks data into categories, has equal intervals between the categories, and has a true zero point. For example, a scale of weight in kilograms.
In the question, respondents are classified only on the basis of gender. This means that the data is categorical and there is no natural ordering of the categories. Therefore, the correct answer is B. Nominal scale.