I have tried this query , which is working

;with cte

as (select activitycode

,activityname

,modifiedon

,row_number() over(order by cast(datediff(minute,0,modifiedon)

as bigint) *60 +datepart(second,modifiedon) desc ,activityname desc )

as rn

FROM #activities AC

INNER JOIN #activitiescategories ACG

ON Ac.activityCategoryIncId = ACG.activityCategoryIncId AND Ac.activityCategorySqlId = ACG.activityCategorySqlId

where activityCategoryCode in ('EAS-31A1'

,'EAS-31A2'

)

)

select activitycode

,activityname

,modifiedon

from cte

where rn=1