The core concept tested here is the understanding of the word 'invaluable'. The correct answer is B, 'Extremely valuable', because 'invaluable' means something is so valuable that its value cannot be measured. Option A, 'Not valuable', is incorrect as it is the opposite of the true meaning. Option C, 'Of little value', and Option D, 'Valueless', are also incorrect for the same reason. The key to solving this question is knowing the definition of 'invaluable' and recognizing its positive connotation.