In order for anyone to offer valuable feedback, we ALWAYS need to know what product versions you are using, which Operating Systems are involved, client side information (browser, jre, os, etc), and if an error is involved, what are the details of the error. It is also helpful to tell us what you have tried already.
Finally, and specific to your question, what do you mean by "mark"? You can color code the current row by creating a Visual Attribute then set the block property for "Current Record Visual Attribute Group".
There is no grid in use in forms
however you were using the ORACLE spreadsheet in the OCX item before 10g in forms5.
you can use the coloring property of the ORACLE spreadsheet if you are using old forms.
If you are talking about the text items shown in the multile instances means with more display records. then your can use the Visual Attirbute
and can assign to the block the current Record Attribute to the visual attribute which you made now.