b. Let cost(b,e) = 4. Now, we should replace (d, e) with (b, e) in MST
c. Let cost(e, f) = 4. Now, we should replace (d, e) with (e, f) in MST
d. Let cost(a, d) = 3. Now, we should replace (d, e) with (a, d) in MST (actually cost(a,d) ≥ 5)
e. Let cost(b, c) = 3. Now, we can replace (c, f) with (b, c) in MST
a. cost(a,b) ≥ 3, won't affect the MST.