To solve this question, the user needs to have knowledge of basic programming and shell scripts.

Option A is incorrect because the variable $tree is not enclosed in quotes, which will result in a syntax error. The correct way to assign a string value to a variable in PHP is to enclose it in quotes. Thus, the correct syntax for option A is $tree = "maple";echo $tree

Option B is incorrect because the variable tree is not preceded by a dollar sign $, which will result in a syntax error. Thus, the correct syntax for option B is $tree = "maple"; echo $tree

Option C is incorrect because the export command is used to set an environment variable so that it can be accessed by child processes. In this case, the sh -c command creates a new shell process and prints the value of the environment variable $tree. However, since the variable was not exported, it will not be accessible by the new shell process, and the output will be empty. Thus, the correct syntax for option C is tree=maple; export tree; sh –c "echo $tree"

Option D is correct because it correctly assigns the string value "maple" to the variable tree, exports it so that it can be accessed by child processes, and then prints its value using the sh -c command. Thus, the correct answer is:

The Answer is: D