Take this with a grain of salt but I think it should be 3s.f. because (in physics at least), you assume the most amount of s.f. in the number with the least possible amount of s.f.
E.g. between 100, 25 and 0.173,
100 has max 3 and min 1
25 has 2
0.173 has 3
So we take bare min: 1 which is...