I am filling the W7 form and am stuck at 6c. It asks me for visa type, Visa number and expiration Date. I am not clear as to which number would mean the US Visa number. Please let me know. Thanks.

They want the control number in the upper right hand corner of the visa. It's about 14 digits long.

I found it to be something else.

VISA NUMBER: The number found in the lower right corner of the visa stamp/sticker in your passport, usually written in red.

Please correct me, if I am wrong.

Regards
Manish

Since I answered your question, I got clarification from the IRS.

Put BOTH numbers (the control number AND the Red Visa number) on Line 6c.