The correct combination is (1) A, B and C.
Statement D is wrong because viral P19 is a suppressor of RNA silencing, not an activator of RISC.

Checking each statement

A. “It is driven by siRNA derived from double-stranded viral RNA.” – True

Viral replication often produces double-stranded RNA, which is processed by Dicer-like enzymes into 21–24 nt siRNAs that guide antiviral RNA silencing.​
These siRNAs are the core triggers of plant RNA interference against viruses.

B. “siRNA requires RISC for its function.” – True

siRNAs must be loaded into the RNA-induced silencing complex (RISC), whose core is an Argonaute protein that uses the siRNA as a guide to recognize and cleave complementary viral RNAs.​
Without RISC loading, siRNAs cannot execute silencing.

C. “Many plant viruses encode proteins that act as suppressors of RNA silencing.” – True

Numerous plant viruses produce RNA silencing suppressor (RSS) proteins—for example HC-Pro, p19, p21—that interfere with different steps of the silencing pathway (dicing, RISC loading, systemic spread).​

D. “Viral P19 protein activates RNA induced silencing complex.” – False

Tombusviral P19 is a well-characterized suppressor of RNA silencing: it forms dimers that bind and sequester siRNA duplexes, preventing their incorporation into RISC and thereby blocking antiviral silencing.​
So P19 does the opposite of activation; it inhibits RISC assembly.

Why option (1) is correct

• Statements A, B, and C accurately describe core features of antiviral RNA silencing and viral counter-defenses.

• Statement D mischaracterizes P19, so it must be excluded.
  Thus, the correct combination is A, B and C.