This line can only be iambic, because trochaic tetrameter (perhaps being too recent a form to have developed conventional variations) does not tolerate inversion, internal catalexis or hypermetricality:
Here catalexis has the incidental extra effect of clarifying the grammar: the syntactic break it draws attention to shows over empty chairs to be an adverbial modifying the verb spreads, rather than a postmodifier of the noun lights.
The reading increase is Larkin's (on tape); the more usual increase would create a second catalexis (the brackets indicate reversal of stress pattern under the Rhythm Rule).
But if we assume that the catalexis is initial, the positions of the caesura and bridges line up exactly and we find an understanding for how these two meters could be freely mixed in with one another.
This makes it even more marked than the trimeter since the unmarked case (acatalectic dimeter) is to have two metra to the line and no catalexis.
Candidate (a) has final catalexis and consistent violation of NOLAPSE, as desired, but removing the final metrical position leaves an L as last position, in violation of ANCEPS.
1) and then review our proposals for understanding line length, rhythm, and catalexis in Greek meters and beyond (section 6.
Why do we find catalexis once per line rather than once per verse foot?
Suppose that catalexis applied to the beginning of every verse foot in tetrameter instead of just the line-initial verse foot.
We begin with the simplest proposal, that catalexis is a marked state and that the markedness can be understood as violation of a constraint (FILL) that requires all metrical positions in a line to be filled with text.
Consider what happens if we scan the following line with initial catalexis rather than final catalexis.
Thus if catalexis is initial in the anapestic tetrameter, the end of a line is basically dactylic (looking at the penultimate foot) or spondaic (looking at the last).