The correct answer is option 'C' - Striven.

Explanation:
The verb 'strive' is an irregular verb, which means that its past participle form does not follow the regular pattern of adding '-ed' to the base form. Instead, it has its own unique past participle form 'striven'.

To understand the past participle form of 'strive', let's look at the different forms of the verb:

- Base form: strive
- Present participle: striving
- Simple past: strove
- Past participle: striven

The base form 'strive' means to make great efforts or struggle to achieve something. The present participle 'striving' is used to describe ongoing or continuous efforts. The simple past form 'strove' is used to describe actions that happened in the past.

The past participle form 'striven' is used in combination with helping verbs to form perfect tenses, passive voice, and other verb forms. For example:

- Present perfect tense: I have striven to improve my skills.
- Past perfect tense: She had striven to overcome her fears before the competition.
- Passive voice: The issue has been striven against by many activists.
- Continuous forms: He was striven to accomplish his goals.

It's important to note that the past participle form 'striven' is not commonly used in everyday conversation. The simple past form 'strove' is more commonly used when talking about past actions. However, in formal or written contexts, the past participle form 'striven' is used to convey a stronger sense of effort or struggle.

In conclusion, the correct past participle form of the verb 'strive' is 'striven'.