C

Snellen letters are designed so that, at the distance for which a line is calibrated, the whole letter subtends 5 minutes of arc and each limb 1 minute of arc [van Gijn & Gijselhart, 2012, PMID 22510417].  
The biggest line on a standard chart is the 6/60 (20/200) line; its letters are 10 × larger than the 6/6 letters, so the visual angle at 6 m is 10 × 5 minutes = 50 minutes of arc.