What is a percentage of endothelial cell loss during Descemet's stripping automated endothelial keratoplasty –
Endothelial cell loss is a known phenomenon of endothelial keratoplasty (including DSAEK).
I am not sure about the answer as various studies have given different values at a different time (post-operatively).
First read following statements which are supporting my answer:-

"Current modifications of EK such as DSAEK have shown a significantly higher cell loss at just 6-12 months after DSAEK surgery".

–Corneal endothelial transplant

"Reports of endothelial cell loss have been very variable from as high as 50% at 6 months to 26% after 2 years in the Busin series. Other reports suggest that there is no difference between cell loss with a 40% loss at 1 year for both PLK and PK and a recent study comparing endothelial cell loss in historic PK Vs DSAEK or other PLK techniques showed no measurable difference".

–Cornea & External eye disease

"The 12-month cell loss for 2 diagnoses (Pseudophakic/aphakic corneal edema and Fush's dystrophy) was comparable, i.e. 41% and 37%.

–Internet

"One-year endothelial cell loss after DSAEK is 15 - 40% (average 27%)".

—XXIV Annual Meeting European Eye Bank Association

"There have been several publications showing endothelial cell loss to be in the range of 24-50% at 12-24 months post DSAEK". 

Now read the statements, which have different values at different post-operative period:

"The 1, 3, and 5-year endothelial cell losses derived from the models after DSAEK were 56%, 66%, and 73%, respectively and after THPK were 24%, 50% and 64% respectively". 
"The 5-year endothelial cell loss after DSEK compared favourably with that measured after penetrating Keratoplasty in the cornea donor study (53% Vs 70%)".
"At 1-year post DSAEK, Bahar et al. published 36% cell loss, Busin et al. reported 24% cell loss; Covert and Koening published 57% and Gorovoy et al. 40%. At 2 years, the rate of endothelial cell loss is between 41% (Price & Price) and 36% (Terry et al). 

However, amongst the given options, option 'c' fits best in all the data mentioned above.
0 – 5%
10 – 20%
30 – 40%
50 – 60%