Correct Answer: scarcely

Solution : The correct choice is the fourth option.

Explanation:
The reason "scarcely" is the most appropriate option is that it emphasises the difficulty or near-impossibility of believing the speaker's statement. "Scarcely" adds a sense of incredibility, aligning well with the context where the speaker is expressing that what he saw is hard to believe.

The meanings of the other options are as follows:

• Seldom: Infrequently or rarely occurring.
• Often: Frequently or regularly occurring.
• Rarely: Not occurring often; infrequently happening.

Therefore, the complete sentence would be: You would scarcely believe me when I tell you what I saw.