If $2^a+2^b=3^c+3^d$, the number of integers $a,b,c,d$ which can possibly be negative, is, at most:
$\textbf{(A)}\ 4 \qquad \textbf{(B)}\ 3 \qquad \textbf{(C)}\ 2 \qquad \textbf{(D)}\ 1 \qquad \textbf{(E)}\ 0$
Assume $c,d \ge 0$, and WLOG, assume $a<0$ and $a \le b$.  This also takes into account when $b$ is negative.  That means
\[\frac{1}{2^{-a}} + 2^b = 3^c + 3^d\]
Multiply both sides by $2^{-a}$ to get
\[1 + 2^{-a+b} = 2^{-a} (3^c + 3^d)\]
Note that both sides are integers.  If $a \ne b$, then the right side is even while the left side is odd, so equality can not happen.  If $a = b$, then $2^{-a} (3^c + 3^d) = 2$, and since  $a<0$, $a = -1$ and $3^c + 3^d = 1$.  No nonnegative value of $c$ and $d$ works, so equality can not happen.  Thus, $a$ and $b$ can not be negative when $c,d \ge 0$.
Assume $a,b \ge 0$, and WLOG, assume $c < 0$ and $c \le d$.  This also takes into account when $d$ is negative.  That means
\[2^a + 2^b = \frac{1}{3^{-c}} + 3^d\]
Multiply both sides by $3^{-c}$ to get
\[3^{-c} (2^a + 2^b) = 1 + 3^{d-c}\]
That makes both sides integers.  The left side is congruent to $0$ modulo $3$ while the right side is congruent to $1$ or $2$ modulo $3$, so equality can not happen.  Thus, $c$ and $d$ can not be negative when $a,b \ge 0$.
Assume $a,c < 0$, and WLOG, let $a \le b$ and $c \le d$.  This also takes into account when $b$ or $d$ is negative.  That means
\[\frac{1}{2^{-a}} + 2^b = \frac{1}{3^{-c}} + 3^d\]
Multiply both sides by $2^{-a} \cdot 3^{-c}$ to get
\[3^{-c} (1 + 2^{b-a}) = 2^{-a} (1 + 3^{d-c})\]
That makes both sides integers.  The left side is congruent to $0$ modulo $3$ while the right side is congruent to $1$ or $2$ modulo $3$, so equality can not happen.  Thus, $a$ and $c$ can not be negative.
Putting all the information together, none of $a,b,c,d$ can be negative, so the answer is $\boxed{\textbf{(E)}}$.