Menthol is (1R,2S,5R)-2-isopropyl-5-methylcyclohexan-1-ol (OH on C1, isopropyl on C2, CH${}_3$ on C5). Which of the following stereoisomers can adopt a chair in which all three substituents are equatorial?

All-equatorial requires OH/iPr trans (1,2), OH/CH${}_3$ cis (1,3-relationship via C1 and C5) and iPr/CH${}_3$ trans (1,4-relationship). Translating the descriptors into up/down relationships (from CIP assignments): (1R,2S,5R) satisfies all three; (1S,2S,5R) has OH cis to iPr; (1R,2S,5S) has OH trans to CH${}_3$; (1R,2R,5R) has all three cis. So only (1R,2S,5R), natural menthol, is all-equatorial.