Explanation
OCL is the most suitable and compatible language for placing formal pre-conditions and post-conditions on an activity diagram. OCL is part of the UML standard and can be integrated withSysML without any conflicts or inconsistencies. OCL can express complex logical expressions that can check the state of the system before and after an activity is executed. BPMN is a business process modeling language that is not compatible with SysML. English is a natural language that may not be precise or unambiguous enough for formal specifications. OWL is a web ontology language that is not designed for modeling activities or constraints.
VSL is a value specification language that can only express simple values and expressions. XMI and XML are markup languages that are used for exchanging models between tools, not for specifying constraints on models. References: OMG-Certified Systems Modeling Professional - Model Builder - Advanced (OCUP2-ADV) Examination Guide Version 1.0, Section 4.3