The correct preposition to fill in the blank is "from" (option C).

Explanation:

- The term "preposition" refers to a word that shows the relationship between a noun or pronoun and other words in a sentence. In this case, the preposition is used to indicate the cause or source of the suffering experienced by the elderly.
- The phrase "suffer from" is a common idiom that is used to describe the experience of being affected by a particular condition or problem.
- When we say that the elderly suffer "from" general weakness, it means that they experience or endure general weakness as a result of various factors such as age-related decline, chronic illnesses, or other health conditions that are commonly associated with old age.
- The preposition "from" is used in this context to indicate the source or cause of the suffering. It suggests that the general weakness originates or arises from certain factors or conditions that affect the elderly.
- The prepositions "with" (option A) and "by" (option B) are not appropriate in this context. While they can be used in other contexts, they do not convey the same meaning as "from" in relation to suffering. "With" is generally used to indicate the presence of something, while "by" is often used to express the means or method by which something happens.
- The preposition "of" (option D) is also not suitable in this context. "Of" is used to indicate possession, origin, or association, and it does not convey the same meaning as "from" in relation to suffering.

In conclusion, the correct preposition to fill in the blank is "from" (option C), as it accurately conveys the source or cause of the general weakness experienced by the elderly.