Published a visual tool to show how deadlock conditions can be modeled using directed graphs.

The term deadlock describes a situation in which two computer programs which are sharing the same resource are effectively preventing each other from accessing the resource. The result is both programs are ceasing to function.
Holt published a visual tool to show how deadlock conditions can be modeled using directed graphs. (year 1972)
Holt's graphs have two kinds of nodes: processes ( circles) and resources (squares).

Ithink the answer is letter d

In 1973 was when graphical user interfaces (guis) pioneered.