Which of the following relational query languages have the same expressive power?

what is the meaning of "restricted to safe expressions" ?

@Bikram sir

@ set2018

A safe expression means an expression which must always produce a finite number of tuples as its result .

see these for example : http://people.cs.pitt.edu/~chang/156/10calculus.html

http://www.w3schools.in/dbms/relational-calculus/

Relational algebra has same expressive power as relational calculus (tuple relational calculus and domain relational calculus) and these all are relational query languages.

so option D is true