Implementing logic expression and the truth table of logic function
You have probably learned, that you can build every logic gate using NAND gates. This is often done, because NAND gates are easy to build. 

In d), the task is to rebuild this circuit, only using NAND gates.

In e) the task is to find the critical (aka the longest) path in your circuit and calculate it's propagation delay, by using the given propagation delay of single gate.