Which of the following search strings is considered invalid?

The search string in option C is invalid due to its syntax in the context of using the chart command. The chart command requires fields to be specified in a way that is compatible with its parameters. In this case, the correct form for specifying multiple fields requires the use of the by keyword to delineate how the count should be aggregated.

While "count over host, status" appears somewhat logical, it is not the proper syntax for how the chart command operates. The correct syntax for referencing multiple fields when using the chart command should always use "by" to define how the results are grouped. Therefore, the correct usage would resemble "count by host, status" or "count by status, host" rather than combining the terms with "over".

The other options utilize valid syntax specific to the chart command, allowing for fields to be properly grouped and counted. Each of them adheres to the expected parameter structure, ensuring they can produce meaningful results in Splunk.