What follows in the sequence: a, b, __, d ?

The sequence provided consists of letters from the English alphabet. Analyzing the order of the letters, the sequence starts with "a" followed by "b." The next letter in the sequence logically follows the alphabetical order, which would be "c." Finally, "d" comes after "c." Therefore, the correct answer is "c," which fits sequentially between "b" and "d."

This makes sense when considering the overall structure of the English alphabet, where recognizing patterns is key in mental ability tests.