I habe Micromax 42C0050UHD 42 Inch LED 4K TV with digital audio out provision. I want to connect that with 5.1 samsung home theater having optical audio in provision.
Please guide which cable to purchase
There is no passive cable that can convert an electrical signal to an optical signal.
If the TV has a single RCA jack that is labeled digital audio output you will need an active coax to optical digital audio converter. https/www.cablestogo.com/product/40018/coaxial-to-toslink-optical-digital-audio-converter-taa-compliant
and both the cable linked to above and an RCA digital audio cable (a composite video cable usually works too).