  In the present paper under certain conditions the description of maximal
solvable extension of nilpotent Lie superalgebras is obtained. Namely, we
establish that under the condition which ensures the fulfillment of an analogue
of Lie's theorem, a maximal solvable extension of a $d$-locally diagonalizable
nilpotent Lie (super)algebra is decomposed into a semidirect sum of its
nilradical and a maximal torus of the nilradical. In the case of Lie algebras
this result gives an answer to \v{S}nobl's conjecture. We present an
alternative method of describing of the aforementioned solvable Lie
(super)algebras. In addition, a criterion of the completeness of solvable Lie
algebras is established and it is proved that the first group of cohomologies
with coefficient itself for a solvable Lie algebra, vanishes if and only if it
is a maximal solvable extension of a d-locally diagonalizable nilpotent Lie
algebra. Finally, we present an example to illustrate that the result obtained
for the descriptions of maximal solvable extensions of nilpotent Lie
(super)algebras does not hold true for Leibniz algebras.
