  To investigate spin susceptibility in a superconducting (SC) state, we
measured the $^{125}$Te-nuclear magnetic resonance (NMR) Knight shifts at
magnetic fields ($H$) up to 6.5 T along the $b$ and $c$ axes of single-crystal
UTe$_2$, a promising candidate for a spin-triplet superconductor. In the SC
state, the Knight shifts along the $b$ and $c$ axes ($K_b$ and $K_c$,
respectively) decreased slightly and the decrease in $K_b$ was almost constant
up to 6.5 T. The reduction in $K_c$ decreased with increasing $H$, and $K_c$
was unchanged through the SC transition temperature at 5.5 T, excluding the
possibility of spin-singlet pairing. Our results indicate that spin
susceptibilities along the $b$ and $c$ axes slightly decrease in the SC state
in low $H$, and the $H$ response of SC spin susceptibility is anisotropic in
the $bc$ plane. We discuss the possible $d$-vector state within the
spin-triplet scenario and suggest that the dominant $d$-vector component for
the case of $H \parallel b$ changes above 13 T, where $T_{\rm c}$ increases
with increasing $H$.
