Which of the following is not a proposition ?

Options

A. `3` is a prime
B. `sqrt(2)` is irrational
C. Mathematics is interesting
D. 5 is an even integer

Explanation

To determine which of the following options is not a proposition, we first need to understand what a proposition is. A proposition is a statement that can either be true or false, but not both.

Let's analyze the given options one by one:

1. **Option 1: "3 is a prime number."**
- This statement can be evaluated as true because 3 is indeed a prime number. Since it can be classified as true, it qualifies as a proposition.

2. **Option 2: "√2 is irrational."**
- This statement is also true because the square root of 2 cannot be expressed as a fraction of two integers, making it an irrational number. Therefore, this is also a proposition.

3. **Option 3: "The sky is blue."**
- This statement can be true or false depending on the time of day and weather conditions. However, it is still a statement that can be evaluated as either true or false, so it qualifies as a proposition.

4. **Option 4: "What time is it?"**
- This is a question, not a statement that can be evaluated as true or false. Therefore, it does not qualify as a proposition.

Based on the analysis, the option that is not a proposition is **"What time is it?"**

### Steps Summary:
1. Define what a proposition is.
2. Analyze each option to determine if it can be classified as true or false.
3. Identify the option that does not meet the criteria of a proposition.