There is a quite relevant paper in Science with a fine analysis and interpretation on $\delta$ 18O evolution during the Phanerozoic Eon, which quite summarily display how this ratio depend on the sea level among other things.
Full reference here :
Miller, K. G.; Kominz, M. A.; Browning, J. V.; Wright, J. D.; Mountain, G. S.; Katz, M. E.; Sugarman, P. J.; Cramer, B. S.; Christie-Blick, N. & Pekar, S. F. The Phanerozoic Record of Global Sea-Level Change Science, 2005, 310, 1293-1298
Actually, following this paper, sea level was somewhat higher than today during the late Cretaceous, with a 100 $\pm$ 50 m peak during that period (in the abstract). They established the concept of a small and ephemeral ice sheet in Antarctica as well between 100 and 33 Million years. Some significant factors regulating the sea level and oxygen ratios would be, for any given time:
- Sea Level
- Average global temperature
- Tectonic activity and related tectonoeustasy > implying impact on water displacement
Briefly to answer the question, Figure 3 in the paper show the $\delta$ 18O in all time during the Cretaceous as lower than during recent times - so b) is the answer.
Redrawn and simplified after Miller et al. 2005