To create a custom Fixlet, you need to use the Action Script language, which is a set of commands that run on relevant clients to fix the problem identified by the Applicability Relevance clause1.

The Action Script language supports various types of scripts that can be executed on different operating systems, such as Windows, Linux, Mac OS, etc2.

Some of the valid Action Script types are:

waithidden or runhidden, which execute a command or script in the background without showing a window2.

wait or run, which execute a command or script in the foreground and show a window2.

override, which modifies the behavior of a command or script, such as changing the working directory, the user context, the completion timeout, etc2.

Among the options given in the question, Powershell and csh are valid Action Script types, as they are supported by the waithidden, runhidden, wait, and run commands2.

Powershell is a scripting language for Windows systems, which can be executed by using the powershell.exe executable2.

csh is a shell script for Unix-like systems, which can be executed by using the csh command2.

AppleScript, Python, and Javascript are not valid Action Script types, as they are not supported by the Action Script language2.

AppleScript is a scripting language for Mac OS systems, which can be executed by using the osascript command2.

Python is a general-purpose programming language, which can be executed by using the python command2.

Javascript is a scripting language for web browsers, which can be executed by using the cscript.exe or wscript.exe commands on Windows systems2.

References:

The Action Script Language

Action Script Guide