The least 1 digit number that is composite is 4. It is because all numbers before that are prime.
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A prime number is a number with two and only two factors: 1 (also known as unity) and itself. A composite number is a number with more than two factors; in other words, it has at least one factor besides 1 and itself. 12 has factors other than 1 and itself, so it is not a prime number. For example, it is divisible by 3. (You can check this by adding the digits of the number: 1 + 2 =3. If the sum of the digits is divisible by 3, the number is also divisible by 3.) Therefore, 12 is a composite number.
A prime number is a number with two and only two factors: 1 (also known as unity) and itself. A composite number is a number with more than two factors; in other words, it has at least one factor besides 1 and itself. 231 has factors other than 1 and itself, so it is not a prime number. For example, it is divisible by 3. (You can check this by adding the digits of the number: 2 + 3 + 1 = 6. If the sum of the digits is divisible by 3, the number is also divisible by 3.) Therefore, 231 is a composite number.
147 has factors other than 1 and itself, so it is not a prime number. For example, 147 is divisible by 3. (You can quickly check this by adding up the digits of 147: 1 + 4 + 7 = 12. If the sum of the digits is divisible by 3, so is the number.) Therefore, 147 is a composite number.
Yes, 43 is a two-digit odd number because it has two digits and is not divisible by 2. It is a prime number because it can only be divided by 1 and itself. A composite number is a positive integer greater than 1 that has at least one positive divisor other than 1 and itself, so 43 is not a composite number.