Which of the following factors tends to increase the stability of metal ion complexes?

In the isoelectronic series of metal carbonyls,the $CO$ bond strength is expected to increase in the order:

The reaction of nitroprusside anion with sulphide ion gives purple colouration due to the formation of

For the reaction given below:
$CoCl_{3} \cdot xNH_{3} + AgNO_{3(aq)} \rightarrow$
If two equivalents of $AgCl$ precipitate out,then the value of $x$ will be $....$

$50 \, mL$ of $0.2 \, M$ solution of a compound with empirical formula $CoCl_3 \cdot 4NH_3$ on treatment with excess of $AgNO_{3(aq)}$ yields $1.435 \, g$ of $AgCl$. Ammonia is not removed by treatment with concentrated $H_2SO_4$. The formula of the compound is:

What is the color of a cobalt chloride solution?