The ServiceNow Store is a marketplace for free and paid certified ServiceNow applications and integrations.
The ServiceNow Store provides customers with access to Now Certified enterprise workflow apps from partners that complement and extend ServiceNow products and solutions. Customers can browse, try, buy, and deploy apps and integrations that suit their needs and enhance their ServiceNow experience. The ServiceNow Store is not the source for ServiceNow Community created developer content, as that is available on the Developer Portal or the Share site. The ServiceNow Store is not a downloadable content ServiceNow script archive, as that is available on the Script Library or the Script Repository. The ServiceNow Store is not an alternate name for the ServiceNow Developer Share site, as that is a separate site where developers can share applications, code snippets, UI pages, etc. Reference: ServiceNow Store

"There are two possible ways to respond to events:
- Email Notification
- Script Action" - see this quote in link below:
https://developer.servicenow.com/dev.do#!/learn/learning-plans/tokyo/new_to_servicenow/app_store_learnv2_automatingapps_tokyo_responding_to_events

The correct combination of statements is a, b, and d. These are possible reasons to build custom applications on ServiceNow:
To replace outdated, inadequate, custom business applications and processes. Building custom applications on ServiceNow can help digitize and automate manual or legacy processes that are not covered by existing ServiceNow solutions. This can improve efficiency, data quality, user experience, and innovation.
To extend service delivery and management to all enterprise departments. Building custom applications on ServiceNow can help provide consistent and scalable services across different functions and teams in the organization. This can enhance collaboration, visibility, productivity, and customer satisfaction.
To extend the value of ServiceNow. Building custom applications on ServiceNow can help leverage the capabilities and benefits of the Now Platform, such as low-code development tools, workflow automation engine, AI-powered insights, security operations, etc. This can increase agility, resilience, performance, and value.
The statement c is not a valid reason to build custom applications on ServiceNow:
To allow users full access to all ServiceNow tables, records, and fields. Building custom applications on ServiceNow does not imply granting users full access to all data and objects in ServiceNow. Access control rules still apply to custom applications and their components to ensure security and compliance. Reference: Build Custom Apps in ServiceNow - eBook

https://www.servicenow.com/community/it-service-management-forum/email-notification/m-p/695221

See list of triggers on right hand side of this webpage: https://docs.servicenow.com/en-US/bundle/tokyo-application-development/page/administer/flow-designer/reference/flow-triggers.html The trigger types in Flow Designer are Application, Record, Schedule, and Topic. Outbound Email is not a trigger type, but an action type that can be used in a flow to send an email message1. References: Flow Designer Trigger Types