$(strlen(s)- strlen(t)) = 3-5 =-2$
But in C, when we do operation with two unsigned integers, result is also unsigned. ($strlen$ returns $size\_t$ which is unsigned in most systems). So, this result "$-2$" is treated as unsigned and its value is $INT\_MAX - 2$ (not sure about all systems, but at least on systems using $2$'s complement representation). Now, the comparison is between this large number and another unsigned number $c$ which is $0$. So, the comparison return TRUE here.
Even if '$c$' is declared as "$int$", while doing an operation with an unsigned int, it gets promoted to unsigned int and we get the same result.
Hence $(strlen(s)- strlen(t)) >0$ will return $1$ on execution thus the conditional operator will return the true statement which is $strlen(abc) =3$.
Ans should be $3$.