What is wrong with the following function definition: function Hello();{ alert("Hello"); document.write("Hello on the screen"); }?

The presence of the semicolon after Hello() in the function definition is problematic because it effectively terminates the function declaration prematurely. In JavaScript, when defining a function, the syntax requires a particular structure where the function name is followed by parentheses and, immediately after that, an opening curly brace without an intervening semicolon. This closing curly brace denotes the start and end of the function's body.

When a semicolon is included after the function name and parentheses, it makes the interpreter think that the function definition is complete at that point, and the subsequent block of code enclosed in curly braces is not recognized as part of the function. As a result, you will encounter a syntax error when this code is executed, since the function’s intended actions won't be encapsulated properly within its definition.

Other options may address common issues that can occur while writing functions, but they do not directly relate to the fundamental syntax error caused by the incorrect placement of the semicolon, which is the core issue in this function definition.