What is the best course of action when receiving an unsolicited part-time job offer via WhatsApp or SMS from an unknown source?

When you receive an unsolicited offer for a part-time job via WhatsApp or SMS from an unknown source, it's crucial to exercise caution to avoid potential scams. The best course of action involves two key steps:

• Validate the credibility of the company offering the job: Research the company independently. Check their official website, look for reviews, and verify their contact information. Be wary if you can't find legitimate information or if the details seem inconsistent.
• Be cautious before initiating any financial transaction: Legitimate job offers rarely ask for money upfront. If you are asked to pay a fee for registration, training, or any other reason, it is a major red flag for a scam.

The option "Option (a) and (b)" refers to combining these two essential precautions. Assuming 'a' refers to "Validate the credibility of company offering job" and 'b' refers to "Be cautious before initiating financial transaction", this combined option provides the most comprehensive and safest approach.

The final answer is $\text{Option (a) and (b)}$.