Consider the following SQL query

Select distinct $a_1, a_2, …, a_n$

from $r_1, r_2, …, r_m$

where P

For an arbitrary predicate P, this query is equivalent to which of the following relational algebra expressions?

Cross product (x) combines the tuples of the one relation with all the tuples of other relation. Thus, tuples of relation $r_1, r_2 .... m$ are combined. Select operator $\sigma$ is used to select resultant tuples. Projection operator $ \pi _{a_1,a_2...a_n}$ is sued to select as subset of attributes from the resultant tuples by specifying the names of the attributes.

So attributes $a_1, a_2, $ an are projected from the resultant tuples.

Thus, option(A) is correct