By the Triangle Inequality, the sum of the lengths of two shortest sides must exceed that of the third.

Case 1:  is the greatest of the three sidelengths. 

Then 

Case 2:  is not the greatest of the three sidelengths - that is, 11 is.

Then , or, equivalently, .

Therefore, .

If two sides are 4 and 8, then the third side must be greater than  and less than . This means  can be 5, 6, 7, 8, 9, 10, or 11. 

The triangle can exist by the Triangle Inequality, since the sum of the two smaller sides exceeds the greatest:

To determine whether the triangle is acute, right, or obtuse, add the squares of the two smaller sides, and compare the sum to the square of the largest side.

Since this sum is greater, the triangle is acute.

If these are the measures of the interior angles of a triangle, then they total . Add the expressions, and solve for .

One angle measures  The others measure:

Since the largest angle measures greater than , the angle is obtuse, and the triangle is as well. Since the three angles each have different measure, their opposite sides do also, making the triangle scalene.

The three sides of a scalene triangle have different measures, so 15 can be eliminated.

By the Triangle Inequality, the sum of the lengths of the two smaller sides must exceed the length of the third side. Since , 8 violates this theorem; since , 22 does as well. 

10 is a valid measure of the third side, since ; it makes all three segments of different length, so it is the correct choice.

By trial and error, we get four ways to add distinct primes to yield sum 33:

In each case, however the Triangle Inequality is violated - the sum of the two shortest lengths does not exceed the third. 

No triangle can exist as described.

A scalene triangle has three sides of different lengths, so we are looking for three distinct prime integers whose sum is a prime integer. 

One of the sides cannot be 2, since the sum of 2 and two odd primes would be an even number greater than 2, a composite number. Therefore, beginning with the least three odd primes, add increasing triples of distinct prime numbers, as follows, until a solution presents itself:

 - incorrect

 - correct

The correct answer, 19, presents itself quickly.

A scalene triangle has three sides of different lengths, so we are looking for three distinct prime integers whose sum is 47. 

There are ten ways to add three distinct primes to yield sum 47:

By the Triangle Inequality, the sum of the lengths of the shortest two sides must exceed that of the greatest. We can therefore eliminate all but four:

The greatest possible length of the longest side is 23.

The three sides of a scalene triangle have different measures. One measure  cannot have is 12, but this is not a choice.

It cannot be true that . Since the perimeter is 

, we can find out what other value can be eliminated as follows:

Therefore, if , then , and the triangle is not scalene. 9 is the correct choice.

We are looking for ways to add three primes to yield a sum of 43. Two or all three (since an equilateral triangle is considered isosceles) must be equal (although, since 43 is not a multiple of three, only two can be equal).

We will set the shared sidelength of the congruent sides to each prime number in turn up to 19:

By the Triangle Inequality, the sum of the lengths of the shortest two sides must exceed that of the greatest. We can therefore eliminate the first three. , , and  include numbers that are not prime (21, 15, 9). This leaves us with only one possibility:

 - greatest length 19

19 is the correct choice.