Correct answer: 288 pm
In a face-centred cubic (FCC) unit cell, the relation between edge length (a) and the radii of cation (r+) and anion (r-) is given by: a = 2√2(r+ + r-). Substituting a = 508 pm and r+ = 100 pm, we solve for r-: 508 = 2√2(100 + r-), leading to r- = 288 pm.