Which neighbor-discovery message type is used to verify connectivity to a neighbor when the linklayer address of the neighbor is known?

IPv6 Neighbor Solicitation Message
A value of 135 in the Type field of the ICMP packet header identifies a neighbor solicitation
message. Neighbor solicitation messages are sent on the local link when a node wants to
determine the link-layer address of another node on the same local link (see the figure below).
When a node wants to determine the link-layer address of another node, the source address in a
neighbor solicitation message is the IPv6 address of the node sending the neighbor solicitation
message. The destination address in the neighbor solicitation message is the solicited-node
multicast address that corresponds to the IPv6 address of the destination node. The neighbor
solicitation message also includes the link-layer address of the source node.
Figure 1. IPv6 Neighbor Discovery: Neighbor Solicitation Message

Reference. http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6_basic/configuration/xe-3s/ip6bxe-3s-book/ip6-neighb-disc-xe.html