How to read data sheet to get the time diagram of the outputs
You've got it. On the rising edge of the clock (assuming the control inputs are properly set up) the four data lines (A,B,C,D) are transferred to the four corresponding outputs (QA,QB,QC,CD).

A,B,C,D are the four Parallel Data Inputs, pins 3-6.


  Why are the inputs for the load are a, b, c and d ?


Umm. I'm not sure what you're asking. They have to be something, right? Why not A-D? Usually data lines are referred to by alphabet letters starting with A. It's a convenient convention. Sometimes data lines, particularly for large data busses, will be called D0, D1, D2, D3, etc. But the IC you're looking at is old, and when digital ICs first started out they were relatively small and simple, so using letters rather than numbers was not an issue.