Step 1: Rule out the wrong options one by one. Potash mainly governs fruit size, colour and shelf life, so its shortage shows up as poor ripening quality, not tissue death at the blossom end.
Step 2: Nitrogen shortage shows up as stunted plants and pale green leaves across the whole crop, not a localized fruit lesion. Boron shortage causes cracked fruit and corky internal tissue, again a different symptom.
Step 3: Calcium is needed to build strong cell walls in the rapidly expanding fruit tissue. When calcium movement to the fruit tip is restricted, the cells there break down and rot, which is exactly blossom end rot.
\[\boxed{\text{Calcium deficiency}}\]