D is a general statement. All the other statements talk about consonants in Irish phonology, but D is the one that introduces the concept.
D talks about the “pronunciation” of the consonants, B and A also talk about the pronunciation but C talks about the contrast in broad and slender consonants and its implication on the meaning of a word.
So, B and A will follow D.
Notice that D states that “all consonants come in pairs, with one having a "broad" pronunciation and the other a "slender" one”, here it mentions first, the "broad" pronunciation and then the slender pronunciation.
B talks about the pronunciation of "broad" consonants, and A talks about pronunciation of "slender" consonants.
Since broad pronunciation was mentioned first in D and then slender pronunciation, their explanations would also be in the same order.
C talks about the contrast in broad and slender consonants and its implication on the meaning of a word. So, it is the last in the sequence.
The order is DBAC