The green colour produced in the borax bead test of a chromium (III) salt is due to:
B
$$Cr_{2}(B_{4}O_{7})_{3}$$
Correct option is C. $$Cr(BO_{2})_{3}$$
In the borax bead test, metal oxide reacts with a glassy bead to form metaborates.
$$Na_{2} B_{4} O_{7} . 10H_{2}O\xrightarrow {\triangle} Na_{2}B_{4}O_{7} \xrightarrow {\triangle} NaBO_{2} + B_{2}O_{3}$$
$$Cr_{2}O_{3} + B_{2}O_{3} \xrightarrow {\triangle} \underset {Green}{Cr(BO_{2})_{3}}$$.
Hence, option C is correct.