a, b, and d are good ways to find the word that you do not remember. All of them can help you. On the contrary, the option c. is not a good way, because you will see that there are too many words in the dictionary starting by any letter. So, the answer to the question is the option c. "Using the first letter of the word and searching through all the entries under that letter" is the answer to the question", which definitely is not a good technique to find the word that you want.