Which set of letter-sound correspondences requires more instructional time due to difficulty?

The choice that requires more instructional time due to difficulty is indeed correct. The letter-sound correspondences represented by h-/h/ and w-/w/ can be more challenging for learners, particularly young children who are just starting to develop their phonemic awareness. This is because both sounds are produced in a similar manner – both are voiceless and produced using the vocal folds to a lesser degree, which can create confusion.

Furthermore, both letters can be less visually distinct to some learners, especially if they haven't yet fully developed their letter recognition skills. As a result, instruction often needs to include more practice and reinforcement for these sounds to ensure students can differentiate and accurately produce them.

In contrast, the other choices typically involve sounds that are more distinct in both their production and visual representation, making them easier for students to grasp quickly. For example, the correspondences in m-/m/ and n-/n/ are quite distinct, and the same applies to b-/b/ and d-/d/, where the letters themselves have significantly different shapes in the alphabet, aiding recognition. Likewise, f-/f/ and s-/s/ also present relatively clear distinctions in phonemic and letter form. Therefore, the set of correspondences involving h and w stands