One that's too big for you to hold.
The recommended drill bit size for a 15/64 inch hole is a 15/64 inch drill bit.
The recommended size of the drill bit for a 7/16 inch hole is a 7/16 inch drill bit.
For a clearance hole you need a 9/64 drill bit.
A 1/4 inch bit will do it.
A 3/16 inch pilot hole drill bit should be used for installing a 1/4 inch screw into hardwood.
A 6mm drill bit can drill a hole that is approximately 15/64 inches in size.
That size is 8-32. The drill bit for a clearance hole is 11/64, or a #17 drill bit.
The recommended drill bit size for a 5/16 inch screw is 1/8 inch.
If you want the bolt to slide through the hole a 5/16 hole will work well.
The 'R' drill is 0.339 inch, which converts to just a tiny bit less than 11/32 of an inch
An 'O' drill bit is a decimal size, 0.3160 inch, which is between 5/16 and 21/64 on the fractional inch scale.
That's a pretty big drill bit, -it drills a hole just a tad larger than 3 3/8 inch wide.