Examine the frontier orbitals and decide whether each of the following elementary reactions should have a high

Question:

Examine the frontier orbitals and decide whether each of the following elementary reactions should have a high or low activation energy for a four-center broadside-collision reaction path.
(a) H2 + D2 → 2HD;
(b) N2 + O2 → 2NO;
(c) F2 + Br2 → 2FBr;
(d) H2 + C2H4 → C2H6.