This search string is not valid: index=web status=50* | chart count over host,status2. This search string uses an invalid syntax for the chart command. The chart command requires one field after the over clause and optionally one field after the by clause. However, this search string has two fields after the over clause separated by a comma. This will cause a syntax error and prevent the search from running. Therefore, option A is correct, while options B and C are incorrect because they are valid search strings that use the chart command correctly.

The correct answer is D. An argument can be passed to the inner macro by nesting parentheses.
A search macro is a way to reuse a piece of SPL code in different searches. A search macro can take arguments, which are variables that can be replaced by different values when the macro is called. A search macro can also contain another search macro within it, which is called a nested macro. A nested macro can also take arguments, which can be passed from the outer macro or directly from the search string.
To pass an argument to the inner macro, you need to use parentheses to enclose the argument value and separate it from the outer macro argument. For example, if you have a search macro named outer_macro (1) that contains another search macro named inner_macro (2), and both macros take one argument each, you can pass an argument to the inner macro by using the following syntax:
outer_macro (argument1, inner_macro (argument2))
This will replace the argument1 and argument2 with the values you provide in the search string. For example, if you want to pass "foo" as the argument1 and "bar" as the argument2, you can write:
outer_macro ("foo", inner_macro ("bar"))
This will expand the macros with the corresponding arguments and run the SPL code contained in them.
References:
* Search macro examples
* Use search macros in searches