Which number completes the series: (-4) -- (-3) -- 1 -- 10 -- 26 -- 51 -- ...

Let $a_n$ be the $n$-th term of the series.
The differences are:
$a_2 - a_1 = -3 - (-4) = 1$
$a_3 - a_2 = 1 - (-3) = 4$
$a_4 - a_3 = 10 - 1 = 9$
$a_5 - a_4 = 26 - 10 = 16$
$a_6 - a_5 = 51 - 26 = 25$

The sequence of differences is $1, 4, 9, 16, 25, \dots$.
These are perfect squares:
$1 = 1^2$
$4 = 2^2$
$9 = 3^2$
$16 = 4^2$
$25 = 5^2$

Following the pattern, the next difference should be $6^2$.
Next difference $= 6^2 = 36$.

To find the next term ($a_7$), add the next difference to the last given term ($a_6$).
$a_7 = a_6 + 36 = 51 + 36 = 87$.

The next number in the series is 87.

The correct option is 87.