Historically, X company has performed well and this led Clark to believe it would continue to do so in the future.
The behavioral bias here is representativeness bias.
Why it is not availability bias? Refer to the curriculum:
Behaviorally, this is availability bias. The availability bias in this context is also called the recency effect, which is the tendency to recall recent events more vividly and give them undue weight. In such models, if the price of an asset rises for a period of time, investors may simply extrapolate this rise to the future.