Temperature of one mole of helium gas is increased by `1^(@)C.` Hence, the increase in its internal energy will be

Explanation

`C _(v) = (( Delta U )/( Delta T )) _(v) or Delta U = C _(v) Delta T`
But for monoatomic gas, `C _(v) = (3)/(2) R. ` Also, `Delta T = 1 ^(@)` (given). Hence, `Delta U = C _(v) Delta T = (3)/(2) R = (3)/(2) xx 2 = 3`