$2.5$ litre of $1MNaOH$ solution mixed with another $3$ litre of $0.5MNaOH$ solution. Then find out molarity of resultant solution.

The correct option is C $0.73M$
Given,

$2.5L$ $1M$ $NaOH$ mixed with $3L$ $0.5M$ $NaOH$

Initially, Number of moles in $2.5L$, $1M$ $NaOH=2.5\times 1=2.5$moles

Number of moles in $3L$, $0.5M$ $NaOH=3\times 0.5=1.5$ moles

$\Rightarrow$ Total number of moles in final mixture=$2.5+1.5=4.0$ moles

The total volume of solution=$2.5+3=5.5L$

$\Rightarrow$ Molarity of final solution=$\frac{4moles}{5.5L}$

$=0.728M\approx 0.73M$.

Hence, the correct option is $C$