Hi Ketaki,
As Emand Alkurdi said for your use case you have to use a reference selector widget.

As above image “Admission” is my main entity and “Student” is associated entity

As above image i’m accessing the associated entity attribute “Student.Name” in Admission Page by using Reference selector widget.

As above image you can see the Name attribute values of student entity.

As above image the associated entity attributes are listed by using reference selector widget.

Note: Dropdown widget is for Enumeration values .for details
for Reference-Selector : https://docs.mendix.com/refguide/reference-selector
for Enumeration : https://docs.mendix.com/refguide/enumerations
for Drop-Down : https://docs.mendix.com/refguide/drop-down

I hope this will help you.
Thanks.

Reference selector widget is used for associated entity attributes. Dropdowns work on enumeration type attribute. 

Here are details of the reference selector widget. 
https://docs.mendix.com/refguide/reference-selector

Hi,

Drop-down widget using for a static list like ‘enumeration’ for associated attribute, you should use reference-selector.

for more information use the link below:

https://docs.mendix.com/refguide/reference-selector