Here are the corrections for each of the sentences, indicating the underlined parts that need correction:

1. Speak **clearer** because I **hardly** understand the instructions.
**Correction:** **B. clearer**
(The comparative form "clearer" should be changed to "clearly," as we use adverbs to modify verbs.)

2. She is **most** helpful shop I have ever **talked** to.
**Correction:** **A. most**
(It should be "the most helpful shop.")

3. Scientists need to develop solar panels in order to **generating** clean and cheap energy.
**Correction:** **C. generating**
(It should be "to generate" instead of "generating.")

4. **One of** **a** worst diseases **mankind** has ever faced **is** cancer.
**Correction:** **D. a**
(It should be "One of the worst diseases.")

5. Some of the methods **used** in advertising **is** unethical **and** unacceptable **in** today’s society.
**Correction:** **C. is**
(It should be "are" instead of "is," as "methods" is plural.)