https://www.citrix.com/blogs/2014/12/29/netscaler-vlans-demystified/
https://support.citrix.com/article/CTX115575

Explanation
answer: B. An HTTP-ECV monitor with the keyword in the Basic Parameters - Receive String field Short Explanation with reference:
An HTTP-ECV (Extended Content Verification) monitor is a type of monitor that allows the Citrix ADC appliance to check the health and availability of the web servers by sending HTTP requests and verifying the responses. An HTTP-ECV monitor can be configured with different parameters, such as Send String, Receive String, and Reverse, to customize the monitoring behavior. The Send String parameter specifies the HTTP request line that the monitor sends to the server. The Receive String parameter specifies the expected response from the server that indicates a successful probe. The Reverse parameter specifies whether to mark the service as UP or DOWN based on the presence or absence of the Receive String in the response.
In this scenario, the administrator needs to create a custom monitor that will look for a specific keyword response from the website, which will be used to keep services in an UP state. To meet this requirement, the administrator can create an HTTP-ECV monitor with the keyword in the Basic Parameters - Receive String field. This will make the monitor expect the keyword in the response from the server, and mark the service as UP if it finds it. The other options are incorrect, as they either use the wrong parameter or the wrong option for the monitoring task. For example, option A uses the Special Parameters - Receive String field, which is not a valid parameter for an HTTP-ECV monitor. Option C uses the Reverse option, which will make the monitor mark the service as DOWN if it finds the keyword in the response. Option D uses the Basic Parameters - Send String field, which will make the monitor send the keyword to the server, instead of looking for it in the response.
Configuring monitors | NetScaler : How to Configure an HTTP-ECV Monitor on a NetScaler Appliance - Citrix Customer Support : How to Configure Extended Content Verification (ECV) Monitors on NetScaler - Citrix Customer Support